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>

review/SKILL.md.tmpl

@@ -14,7 +14,7 @@ allowed-tools:
   - AskUserQuestion
 ---
 
-{{UPDATE_CHECK}}
+{{PREAMBLE}}
 
 # Pre-Landing PR Review
 
@@ -75,7 +75,7 @@ Follow the output format specified in the checklist. Respect the suppressions
 
 **Always output ALL findings** — both critical and informational. The user must see every issue.
 
-- If CRITICAL issues found: output all findings, then for EACH critical issue use a separate AskUserQuestion with the problem, your recommended fix, and options (A: Fix it now, B: Acknowledge, C: False positive — skip).
+- If CRITICAL issues found: output all findings, then for EACH critical issue use a separate AskUserQuestion with the problem, then `RECOMMENDATION: Choose A because [one-line reason]`, then options (A: Fix it now, B: Acknowledge, C: False positive — skip).
   After all critical questions are answered, output a summary of what the user chose for each issue. If the user chose A (fix) on any issue, apply the recommended fixes. If only B/C were chosen, no action needed.
 - If only non-critical issues found: output findings. No further action needed.
 - If no issues found: output `Pre-Landing Review: No issues found.`