docs(agents): add .agents/docs context-engineering home + Ink-parity loop (#28)

Establishes the committed .agents/docs/ convention (distinct from the
uncommitted docs/ working-notes folder) and seeds the Ink-parity
verification loop:

- ink-parity-loop.md: design spec + reusable /loop prompt (audit →
  test-first fix → codex review → PR → CI → auto-merge, hard codex gate).
- ink-parity.md: pinned Ink reference (v7.0.4, commit 40b3a75) + the
  intentional-divergence allowlist the audit skips.
- parity-ledger.md: working ledger of audit sweeps and confirmed gaps.
- AGENTS.md: "Context Engineering" section documenting the convention.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

AGENTS.md

@@ -14,6 +14,26 @@
 - After completing any task, run `vp run ready` (or `vpr ready`) to verify: lint, type-check, test all packages, and build.
 - Never commit anything under `docs/`. That directory is for local working notes and specs — it must stay out of git.
 
+# Context Engineering
+
+Long-lived design context lives in `.agents/docs/` (committed — distinct from the
+uncommitted `docs/` working-notes folder). Convention, borrowed from rolldown:
+
+- One concept per file; files cross-link with `[[other-doc]]`.
+- If a design doc covers the area you're about to work in, **read it first**.
+- If your change affects a design doc, **update it in the same change**. Docs that drift
+  from reality are worse than no docs.
+- Capture the _why_ — trade-offs considered, alternatives rejected, known pitfalls — not
+  just what the code does.
+
+Current docs:
+
+- `.agents/docs/ink-parity-loop.md` — the Ink-parity verification loop (audit → fix → PR
+  → merge) and its reusable `/loop` prompt.
+- `.agents/docs/ink-parity.md` — pinned Ink reference version/SHA + the
+  intentional-divergence allowlist (differences that are by design, not gaps).
+- `.agents/docs/parity-ledger.md` — working ledger of audit sweeps and confirmed gaps.
+
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