docs(prd): PRD 0028 — git-gate new-branch push scan scope

git-gate's pre-receive scans the full ancestry of a new branch, so the
repo's historical test-fixture findings block every new-branch push
(issue #106). Scope the new-ref scan to incoming commits
(`$new --not --all`) with no loss of coverage, and harden the forward
ssh against hangs.

Refs #106

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/prds/0028-git-gate-new-branch-scan-scope.md

@@ -0,0 +1,170 @@
+# PRD 0028: git-gate new-branch push scan scope
+
+- **Status:** Draft
+- **Author:** didericis-claude
+- **Created:** 2026-05-29
+- **Issue:** #106
+
+## Summary
+
+git-gate's pre-receive hook scans the **entire ancestry** of a *new*
+branch for secrets, so any pre-existing finding in repo history blocks
+every new-branch push. Scope the scan to the commits a push actually
+introduces (`$new --not --all`) so a push is gated on what *it* adds,
+not on what's already on the gate/upstream. Also harden the forward
+`ssh` against hangs. Net: new branches can be pushed through the gate
+again, with no loss of leak-detection coverage.
+
+## Problem
+
+In `git_gate_render_hook()` (`bot_bottle/git_gate.py`) the pre-receive
+hook chooses the gitleaks revision range per ref:
+
+```sh
+if [ "$old" = "$zero" ]; then
+  log_opts="$new"          # new branch: `git log <new>` = the FULL ancestry
+else
+  log_opts="$old..$new"    # existing branch: just the pushed delta
+fi
+gitleaks git --log-opts="$log_opts" ...   # exit 1 if ANY finding in range
+```
+
+For a **new** ref there is no `old` to diff against, so the hook passes
+`$new`, which `git log` expands to every commit reachable from the new
+tip. This repo's history contains 11 deliberately secret-shaped strings
+(demo manifests, `docs/demo.tape`, and the pipelock/sandbox-escape
+integration tests that exist *to exercise* the DLP). gitleaks reports
+`438 commits scanned … leaks found: 11`, the hook `exit 1`s, and the
+push is rejected. Confirmed live against issue #106's bottle: the
+branch never lands in the bare repo and is never forwarded.
+
+Consequence: **no new branch can ever be pushed through git-gate** as
+long as a single historical finding exists — which is permanent.
+
+Two adjacent problems surfaced while diagnosing #106:
+
+- The rejection is **invisible to the client** — over the `git://` +
+  smolmachines forward it presented as a ~75s silent hang, not a
+  `remote: git-gate: gitleaks rejected …` message.
+- The forward `ssh` lacks `BatchMode`/`ConnectTimeout`, so an
+  unreachable upstream or a prompt would hang the hook indefinitely.
+  (Not the cause of #106 — the forward itself works — but a latent
+  hang risk.)
+
+## Goals / Success Criteria
+
+- A new-branch push is scanned **only for the commits it introduces**
+  (reachable from `$new`, not from any ref the gate already has).
+- A new branch that adds no new findings **pushes successfully**, even
+  though historical fixtures still trip a full-history scan.
+- A new branch that *does* introduce a finding is **still rejected**.
+- No reduction in leak coverage for the commits a push actually brings
+  to the upstream (see "Security analysis").
+- Forward `ssh` fails fast (`BatchMode=yes` + `ConnectTimeout`) instead
+  of hanging on a prompt/unreachable upstream.
+- Existing git-gate unit + integration tests stay green; new tests lock
+  the scoped-scan behaviour.
+
+## Non-goals
+
+- **Scrubbing the historical fixture findings.** They're intentional
+  test/demo inputs; scoping the scan resolves the practical problem
+  without rewriting history.
+- **Relaxing the existing-branch path.** `$old..$new` already scans the
+  delta; this PRD only fixes the new-ref branch (and optionally unifies
+  on `--not --all`, see Open questions).
+- **The client-visibility fix is investigation-gated.** Surfacing the
+  rejection over the `git://` + smolmachines path may need separate work
+  (sideband relay); tracked here but may land as a follow-up rather than
+  block the scan-scope fix.
+
+## Design
+
+### Scoped scan
+
+Replace the new-ref range with one that excludes everything the gate
+already knows:
+
+```sh
+if [ "$new" = "$zero" ]; then
+  continue                      # deletion: nothing to scan (unchanged)
+elif [ "$old" = "$zero" ]; then
+  log_opts="$new --not --all"   # new branch: only commits new to the gate
+else
+  log_opts="$old..$new"         # existing branch: the pushed delta
+fi
+```
+
+`git log $new --not --all` = commits reachable from the pushed tip but
+**not** reachable from any ref already in the gate's bare repo.
+
+### Security analysis (why excluding "already-on-the-gate" is safe)
+
+Commits enter the gate's bare repo by exactly two paths:
+
+1. **mirror-fetch from the upstream** — the bare repo is
+   `remote add --mirror=fetch origin`, and the access-hook fetches the
+   upstream before every upload-pack; and
+2. **a push through the gate** — which is gitleaks-scanned before it is
+   forwarded.
+
+Therefore every commit reachable from a gate ref is *already on the
+upstream* or *was already scanned when pushed*. A commit excluded by
+`--not --all` cannot be a new secret arriving at the upstream via this
+push:
+
+- if it's already upstream, re-scanning changes nothing — the content is
+  already there and blocking this branch wouldn't remove it; and
+- if it arrived via an earlier push, it was already scanned.
+
+The only commits that can carry a *new* secret upstream are the ones the
+push introduces — exactly the set `$new --not --all` scans. An agent
+cannot pre-seed a secret commit as "already known" to dodge the scan: it
+can't write refs into the bare repo except by pushing (which scans), and
+the mirror refs come only from the trusted upstream.
+
+**Invariant this relies on:** the bare repo's refs are populated *only*
+by upstream mirror-fetch and gitleaks-gated pushes. That holds in the
+current design (nothing writes refs out-of-band); revisit if that
+changes.
+
+### Forward ssh hardening
+
+Add `-o BatchMode=yes -o ConnectTimeout=<n>` to the hook's `ssh_cmd` so
+a prompt or unreachable upstream fails fast with a clear error instead
+of hanging the receive-pack.
+
+## Implementation chunks
+
+1. **PRD (this commit).**
+2. **Hook scan scope + ssh hardening.** Edit `git_gate_render_hook()`:
+   the new-ref range → `$new --not --all`; add `BatchMode`/`ConnectTimeout`
+   to `ssh_cmd`. Unit tests in `test_git_gate.py` asserting the rendered
+   hook uses the scoped range for new refs and the hardened ssh flags.
+3. **Integration coverage.** A new-branch push carrying no new finding
+   succeeds through a gate whose history contains a fixture finding; a
+   new-branch push that introduces a finding is still rejected.
+4. **(Optional / follow-up) client visibility.** Make a gitleaks/forward
+   rejection reach the client as a `remote:` error over the git:// +
+   smolmachines path.
+
+## Testing strategy
+
+- **Unit (must):** rendered-hook assertions — new-ref uses
+  `$new --not --all`, existing-ref still `$old..$new`, deletion still
+  skipped; ssh_cmd carries `BatchMode=yes` + a `ConnectTimeout`.
+- **Integration (should):** against a real gate seeded with a
+  fixture-bearing history, a clean new branch forwards to the upstream;
+  a new branch with a planted secret is rejected. Skips cleanly on hosts
+  that can't run the bundle (same shape as the existing git-gate
+  integration test).
+
+## Open questions
+
+- **Unify both branches on `--not --all`?** It's also more robust than
+  `$old..$new` for non-fast-forward/force pushes (which can skip commits
+  off the direct path). Tempting to use it for the existing-ref case
+  too; deferred to keep this change tight, but worth a follow-up.
+- **Client visibility mechanism.** Whether the silent-hang is a git
+  daemon sideband-relay issue or specific to the smolmachines forward
+  needs a focused repro before committing to a fix.