feat: Review Readiness Dashboard + gstack-slug helper (v0.5.1) (#130)

* feat: add bin/gstack-slug helper + migrate all inline SLUG computation

Extract the opaque SLUG sed pipeline into a shared 5-line shell script.
Replace 8 inline copies across templates with eval $(gstack-slug).
Sanitizes branch names (/ → -) to prevent subdirectory creation.

* feat: review readiness dashboard — track CEO/Eng/Design reviews per branch

Each review skill logs its result to JSONL. A shared {{REVIEW_DASHBOARD}}
placeholder displays run counts, timestamps, and a CLEARED TO SHIP verdict.
/ship pre-flight reads the dashboard and prompts when reviews are missing.

* chore: bump version and changelog (v0.5.1)

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

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