audit-archived-not-merged: add spec for audit command and shipped stage

[androidand]

What

Add `specsync audit` — a read-only command that cross-references archived OpenSpec changes against GitHub PRs to find archived changes whose PR was never merged.

Changes

• New `specsync audit` command with flags:
  • `-json` for machine-readable output
  • `-fail-on-unmerged` to exit non-zero on unmerged changes
  • `-mark-shipped` to write shipped metadata for merged PRs
• New `shipped` stage — extends the lifecycle: `active → complete → archived → shipped`
• PR matching via specsync marker, branch name convention (`feat/-`), or PR title
• Archived immutability preserved — only `shipped` metadata can override archived stage

Files

• `audit.go` — Audit function, matchPRToChange, isDigits helpers
• `audit_test.go` — comprehensive tests for matching, audit, shipped metadata
• `change.go` — StageShipped constant, refreshState shipped override
• `cmd/specsync/main.go` — runAudit, parseStatusMapping shipped stage
• `README.md` — audit section added
• `site/features.json` — audit feature entry added

Closes #59

[cloudflare-workers-and-pages]

Deploying specsync with    Cloudflare Pages

Latest commit:	5330792
Status:	✅  Deploy successful!
Preview URL:	https://2c8c5401.specsync.pages.dev
Branch Preview URL:	https://audit-archived-not-merged.specsync.pages.dev

View logs