feat: TODOS-aware skills, 2-tier Greptile replies, gitignore fix (#61)

* fix: log non-ENOENT errors in ensureStateDir() instead of silently swallowing

Replace bare catch {} with ENOENT-only silence. Non-ENOENT errors (EACCES,
ENOSPC) are now logged to .gstack/browse-server.log. Includes test for
permission-denied scenario with chmod 444.

* feat: merge TODO.md + TODOS.md into unified backlog with shared format reference

Merge TODO.md (roadmap) and TODOS.md (near-term) into one file organized by
skill/component with P0-P4 priority ordering and Completed section. Add shared
review/TODOS-format.md for canonical format. Add static validation tests.

* feat: add 2-tier Greptile reply system with escalation detection

Add reply templates (Tier 1 friendly, Tier 2 firm), explicit escalation
detection algorithm, and severity re-ranking guidance to greptile-triage.md.

* feat: cross-skill TODOS awareness + Greptile template refs in all skills

/ship Step 5.5: auto-detect completed TODOs, offer reorganization.
/review Step 5.5: cross-reference PR against open TODOs.
/plan-ceo-review, /plan-eng-review: TODOS context in planning.
/retro: Backlog Health metric. /qa: bug TODO context in diff-aware mode.
All Greptile-aware skills now reference reply templates and escalation detection.

* chore: bump version and changelog (v0.3.8)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: update CONTRIBUTING.md for v0.3.8 changes

Clarify test tier cost table (Tier 3 standalone vs combined), add TODOS.md
to "Things to know", mention Greptile triage in ship workflow description.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,27 @@
 # Changelog
 
+## 0.3.8 — 2026-03-14
+
+### Added
+- **TODOS.md as single source of truth** — merged `TODO.md` (roadmap) and `TODOS.md` (near-term) into one file organized by skill/component with P0-P4 priority ordering and a Completed section.
+- **`/ship` Step 5.5: TODOS.md management** — auto-detects completed items from the diff, marks them done with version annotations, offers to create/reorganize TODOS.md if missing or unstructured.
+- **Cross-skill TODOS awareness** — `/plan-ceo-review`, `/plan-eng-review`, `/retro`, `/review`, and `/qa` now read TODOS.md for project context. `/retro` adds Backlog Health metric (open counts, P0/P1 items, churn).
+- **Shared `review/TODOS-format.md`** — canonical TODO item format referenced by `/ship` and `/plan-ceo-review` to prevent format drift (DRY).
+- **Greptile 2-tier reply system** — Tier 1 (friendly, inline diff + explanation) for first responses; Tier 2 (firm, full evidence chain + re-rank request) when Greptile re-flags after a prior reply.
+- **Greptile reply templates** — structured templates in `greptile-triage.md` for fixes (inline diff), already-fixed (what was done), and false positives (evidence + suggested re-rank). Replaces vague one-line replies.
+- **Greptile escalation detection** — explicit algorithm to detect prior GStack replies on comment threads and auto-escalate to Tier 2.
+- **Greptile severity re-ranking** — replies now include `**Suggested re-rank:**` when Greptile miscategorizes issue severity.
+- Static validation tests for `TODOS-format.md` references across skills.
+
+### Fixed
+- **`.gitignore` append failures silently swallowed** — `ensureStateDir()` bare `catch {}` replaced with ENOENT-only silence; non-ENOENT errors (EACCES, ENOSPC) logged to `.gstack/browse-server.log`.
+
+### Changed
+- `TODO.md` deleted — all items merged into `TODOS.md`.
+- `/ship` Step 3.75 and `/review` Step 5 now reference reply templates and escalation detection from `greptile-triage.md`.
+- `/ship` Step 6 commit ordering includes TODOS.md in the final commit alongside VERSION + CHANGELOG.
+- `/ship` Step 8 PR body includes TODOS section.
+
 ## 0.3.7 — 2026-03-14
 
 ### Added