feat: contributor mode, session awareness, recommendation format (#90)

* feat: contributor mode, session awareness, universal RECOMMENDATION format

- Rename {{UPDATE_CHECK}} → {{PREAMBLE}} across all 10 skill templates
- Add session tracking (touch ~/.gstack/sessions/$PPID, count active sessions)
- ELI16 mode when 3+ concurrent sessions detected (re-ground user on context)
- Contributor mode: auto-file field reports to ~/.gstack/contributor-logs/
- Universal AskUserQuestion format: context → question → RECOMMENDATION → options
- Update plan-ceo-review and plan-eng-review to reference preamble baseline
- Add vendored symlink awareness section to CLAUDE.md
- Rewrite CONTRIBUTING.md with contributor workflow and cross-project testing
- Add tests for contributor mode and session awareness in generated output
- Add E2E eval for contributor mode report filing

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

* feat: add Enum & Value Completeness to /review critical checklist

New CRITICAL review category that traces new enum values, status strings,
and type constants through every consumer outside the diff. Catches the
class of bugs where a new value is added but not handled in all switch/case
chains, allowlists, or frontend-backend contracts.

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

* chore: bump v0.4.1, user-facing changelog, update qa-only template and architecture docs

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

* docs: add CHANGELOG style guide — user-facing, sell the feature

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: rewrite v0.4.1 changelog to be user-facing and sell the features

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add evals for RECOMMENDATION format, session awareness, and enum completeness

Free tests (Tier 1): RECOMMENDATION format + session awareness in all
preamble SKILL.md files, enum completeness checklist structure and CRITICAL
classification.

E2E eval: /review catches missed enum handlers when a new status value
is added but not handled in case/switch and notify methods.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add E2E eval for session awareness ELI16 mode

Stubs _SESSIONS=4, gives agent a decision point on feature/add-payments
branch, verifies the output re-grounds the user with project, branch,
context, and RECOMMENDATION — the ELI16 mode behavior for 3+ sessions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: contributor mode eval marked FAIL due to expected browse error

The test intentionally runs a nonexistent binary to trigger contributor
mode. The session runner's browse error detection catches "no such file
or directory...browse" and sets browseErrors, causing recordE2E to mark
passed=false. Override passed to check only exitReason since the browse
error is the expected scenario.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

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

CHANGELOG.md

@@ -1,5 +1,18 @@
 # Changelog
 
+## 0.4.1 — 2026-03-16
+
+- **gstack now notices when it screws up.** Turn on contributor mode (`gstack-config set gstack_contributor true`) and gstack automatically writes up what went wrong — what you were doing, what broke, repro steps. Next time something annoys you, the bug report is already written. Fork gstack and fix it yourself.
+- **Juggling multiple sessions? gstack keeps up.** When you have 3+ gstack windows open, every question now tells you which project, which branch, and what you were working on. No more staring at a question thinking "wait, which window is this?"
+- **Every question now comes with a recommendation.** Instead of dumping options on you and making you think, gstack tells you what it would pick and why. Same clear format across every skill.
+- **/review now catches forgotten enum handlers.** Add a new status, tier, or type constant? /review traces it through every switch statement, allowlist, and filter in your codebase — not just the files you changed. Catches the "added the value but forgot to handle it" class of bugs before they ship.
+
+### For contributors
+
+- Renamed `{{UPDATE_CHECK}}` to `{{PREAMBLE}}` across all 11 skill templates — one startup block now handles update check, session tracking, contributor mode, and question formatting.
+- DRY'd plan-ceo-review and plan-eng-review question formatting to reference the preamble baseline instead of duplicating rules.
+- Added CHANGELOG style guide and vendored symlink awareness docs to CLAUDE.md.
+
 ## 0.4.0 — 2026-03-16
 
 ### Added