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>

plan-eng-review/SKILL.md.tmpl

@@ -14,7 +14,7 @@ allowed-tools:
   - Bash
 ---
 
-{{UPDATE_CHECK}}
+{{PREAMBLE}}
 
 # Plan Review Mode
 
@@ -129,18 +129,15 @@ Evaluate:
 **STOP.** For each issue found in this section, call AskUserQuestion individually. One issue per call. Present options, state your recommendation, explain WHY. Do NOT batch multiple issues into one AskUserQuestion. Only proceed to the next section after ALL issues in this section are resolved.
 
 ## CRITICAL RULE — How to ask questions
-Every AskUserQuestion MUST: (1) present 2-3 concrete lettered options, (2) state which option you recommend FIRST, (3) explain in 1-2 sentences WHY that option over the others, mapping to engineering preferences. No batching multiple issues into one question. No yes/no questions. Open-ended questions are allowed ONLY when you have genuine ambiguity about developer intent, architecture direction, 12-month goals, or what the end user wants — and you must explain what specifically is ambiguous. **Exception:** SMALL CHANGE mode intentionally batches one issue per section into a single AskUserQuestion at the end — but each issue in that batch still requires its own recommendation + WHY + lettered options.
-
-## For each issue you find
-For every specific issue (bug, smell, design concern, or risk):
+Follow the AskUserQuestion format from the Preamble above. Additional rules for plan reviews:
 * **One issue = one AskUserQuestion call.** Never combine multiple issues into one question.
 * Describe the problem concretely, with file and line references.
-* Present 2–3 options, including "do nothing" where that's reasonable.
+* Present 2-3 options, including "do nothing" where that's reasonable.
 * For each option, specify in one line: effort, risk, and maintenance burden.
-* **Lead with your recommendation.** State it as a directive: "Do B. Here's why:" — not "Option B might be worth considering." Be opinionated. I'm paying for your judgment, not a menu.
 * **Map the reasoning to my engineering preferences above.** One sentence connecting your recommendation to a specific preference (DRY, explicit > clever, minimal diff, etc.).
-* **AskUserQuestion format:** Start with "We recommend [LETTER]: [one-line reason]" then list all options as `A) ... B) ... C) ...`. Label with issue NUMBER + option LETTER (e.g., "3A", "3B").
+* Label with issue NUMBER + option LETTER (e.g., "3A", "3B").
 * **Escape hatch:** If a section has no issues, say so and move on. If an issue has an obvious fix with no real alternatives, state what you'll do and move on — don't waste a question on it. Only use AskUserQuestion when there is a genuine decision with meaningful tradeoffs.
+* **Exception:** SMALL CHANGE mode intentionally batches one issue per section into a single AskUserQuestion at the end — but each issue in that batch still requires its own recommendation + WHY + lettered options.
 
 ## Required outputs
 
@@ -192,10 +189,9 @@ At the end of the review, fill in and display this summary so the user can see a
 Check the git log for this branch. If there are prior commits suggesting a previous review cycle (e.g., review-driven refactors, reverted changes), note what was changed and whether the current plan touches the same areas. Be more aggressive reviewing areas that were previously problematic.
 
 ## Formatting rules
-* NUMBER issues (1, 2, 3...) and give LETTERS for options (A, B, C...).
-* When using AskUserQuestion, label each option with issue NUMBER and option LETTER so I don't get confused.
-* Recommended option is always listed first.
-* Keep each option to one sentence max. I should be able to pick in under 5 seconds.
+* NUMBER issues (1, 2, 3...) and LETTERS for options (A, B, C...).
+* Label with NUMBER + LETTER (e.g., "3A", "3B").
+* One sentence max per option. Pick in under 5 seconds.
 * After each review section, pause and ask for feedback before moving on.
 
 ## Unresolved decisions