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/README.md

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+# Decision records
+
+Short, durable records of decisions — one file per decision. This is a
+lightweight [Architecture Decision Record](https://adr.github.io/)
+practice: capture *what was decided and why* in a versioned file so the
+reasoning lives in the clone, not in a forge issue thread or a chat log
+that disappears when the provider does.
+
+See `docs/research/issue-tracking-vs-in-repo-decision-history.md` for
+the rationale behind keeping decision history in-repo.
+
+## When to write one
+
+| Artifact | For |
+|---|---|
+| **PRD** (`docs/prds/`) | A feature: what to build, scope, success criteria. |
+| **Research note** (`docs/research/`) | A landscape/tradeoff investigation. |
+| **Decision record** (`docs/decisions/`) | A decision that isn't itself a feature — a policy, a convention, a "we will / won't do this," or a load-bearing choice made inside a larger PRD that deserves to be discoverable on its own. |
+
+A decision that's fully specified by a PRD doesn't need duplicating
+here. Write a record when the *decision* would otherwise be buried in
+prose, lost in an issue thread, or have no in-repo home at all (small
+requests that don't merit a PRD; non-feature choices like merge
+strategy or a trust posture).
+
+## Format
+
+One Markdown file per decision, numbered sequentially and zero-padded
+(`0001-…`, `0002-…`), matching the PRD numbering style. Keep it short —
+the discipline is writing it down, not the ceremony.
+
+```markdown
+# ADR 0000: <short imperative title>
+
+- **Status:** Proposed | Accepted | Superseded by ADR NNNN
+- **Date:** YYYY-MM-DD
+- **Deciders:** <who>
+
+## Context
+What forced the decision; the constraints in play.
+
+## Decision
+What we decided, stated plainly.
+
+## Consequences
+What follows — the good, and the costs/trade-offs accepted.
+
+## Links
+PRDs, research notes, issues/PRs. Forge links are convenience
+pointers; the reasoning above must stand without them.
+```
+
+## Index
+
+- [ADR 0001](0001-merge-prs-with-rebase.md) — merge PRs with rebase, not merge commits.
+- [ADR 0002](0002-agent-identity-claimed-not-vouched.md) — agent-set git identity is claimed, not vouched.