docs(decisions): add ADR-lite decision log

Add docs/decisions/ with a convention README and back-fill two
decisions that previously had no in-repo home: merging PRs with
rebase (ADR 0001) and the agent-identity claimed-not-vouched trust
posture from PRD 0027 (ADR 0002). Point docs/INDEX.md at it.

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

docs/decisions/0001-merge-prs-with-rebase.md

@@ -0,0 +1,47 @@
+# ADR 0001: Merge PRs with rebase, not merge commits
+
+- **Status:** Accepted
+- **Date:** 2026-05-28
+- **Deciders:** didericis
+
+## Context
+
+PRs need a merge strategy. Gitea offers merge-commit, squash, rebase,
+and rebase-merge. The project uses [Conventional
+Commits](https://www.conventionalcommits.org/) enforced by a
+`commit-msg` hook, and PRDs typically land as a multi-commit PR where
+each commit is meaningful on its own (e.g. PR #95: a `docs(prd)` commit,
+a `feat(manifest)` implementation commit, and a `docs(manifest)`
+commit). The history should stay readable and the individual
+conventional commits should survive onto `main`.
+
+## Decision
+
+Merge PRs with **rebase** (Gitea's `rebase` style; `Do: "rebase"` via
+the API). The branch's commits are replayed onto `main` with no merge
+commit, producing a linear history that preserves each commit verbatim.
+
+## Consequences
+
+- **Linear history**, no merge bubbles; `git log --oneline` reads as a
+  straight sequence of conventional commits.
+- **Each commit is preserved** (unlike squash, which would collapse the
+  PRD/impl/docs commits into one and lose the staged structure).
+- **Commit SHAs are rewritten at merge.** The replayed commits on `main`
+  get new SHAs, and the source branch is deleted, so a link to a file
+  by *branch name* (`/src/branch/<feature>/…`) dies at merge. This is
+  why links to not-yet-merged files are pinned to a **commit SHA**
+  (`/src/commit/<sha>/…`), which stays reachable via the retained
+  `refs/pull/<n>/head` ref. See
+  `docs/research/issue-tracking-vs-in-repo-decision-history.md`.
+- **Trade-off accepted:** without a merge commit, the "these commits
+  landed together as PR #N" grouping is not recorded in git itself — it
+  lives in forge state (the PR). That is a mild concession against the
+  keep-history-in-the-repo posture; the conventional-commit scopes and
+  PRD references in the messages keep changes traceable without it.
+
+## Links
+
+- `docs/research/issue-tracking-vs-in-repo-decision-history.md` — the
+  commit-pinning consequence above.
+- Observed practice: PRs #92, #93 merged with rebase; #95 to follow.