feat: context health soft directive in preamble (T2+)

Adds a "periodically self-summarize" nudge to long-running skills.
Soft directive only — no thresholds, no enforcement, no auto-commit.

Goal: self-awareness during /qa, /investigate, /cso etc. If you notice
yourself going in circles, STOP and reassess instead of thrashing.

Codex review caught that fake precision thresholds (15/30/45 tool calls)
were unimplementable — SKILL.md is a static prompt, not runtime code.
This ships the soft version only.

Changes:
- scripts/resolvers/preamble.ts: add generateContextHealth(), wire into
  T2+ tier. Format: [PROGRESS] ... summary line. Explicit rule that
  progress reporting must never mutate git state.
- All T2+ skill SKILL.md files regenerated to include the new section.
- Golden ship fixtures updated (T4 skill, picks up the change).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

open-gstack-browser/SKILL.md

@@ -374,6 +374,21 @@ AI makes completeness near-free. Always recommend the complete option over short
 
 Include `Completeness: X/10` for each option (10=all edge cases, 7=happy path, 3=shortcut).
 
+## Context Health (soft directive)
+
+During long-running skill sessions, periodically write a brief `[PROGRESS]` summary
+(2-3 sentences: what's done, what's next, any surprises). Example:
+
+`[PROGRESS] Found 3 auth bugs. Fixed 2. Remaining: session expiry race in auth.ts:147. Next: write regression test.`
+
+If you notice you're going in circles — repeating the same diagnostic, re-reading the
+same file, or trying variants of a failed fix — STOP and reassess. Consider escalating
+or calling /checkpoint to save progress and start fresh.
+
+This is a soft nudge, not a measurable feature. No thresholds, no enforcement. The
+goal is self-awareness during long sessions. If the session stays short, skip it.
+Progress summaries must NEVER mutate git state — they are reporting, not committing.
+
 ## Repo Ownership — See Something, Say Something
 
 `REPO_MODE` controls how to handle issues outside your branch: