feat(release): changelog fragments directory + build script (issue #222)

[Delqhi]

Implements the Linux-kernel / Python / Rust pattern for conflict-free changelogs.

What ships:

• CHANGELOG.d/_template.md: per-PR fragment format spec
• CHANGELOG.d/_unreleased.md: migration of the existing
  [Unreleased] block (770 lines) into a fragment
• scripts/build_changelog.sh: aggregator that sorts fragments
  by PR number, prepends the static header, and writes a
  deterministic CHANGELOG.md. --check flag for pre-commit.
• scripts/pre-commit-changelog-check.sh: wrapper for the
  git pre-commit hook (install via symlink)

Acceptance criteria (from #222):

• build_changelog.sh produces byte-identical CHANGELOG.md
  from the existing [Unreleased] block (verified by
  --check exit 0)
• Pre-commit hook refuses a CHANGELOG.md that drifts
  from CHANGELOG.d/ (verified by --check exit 1 on
  dirty CHANGELOG.md)
• Two simultaneous PRs each adding one fragment can be
  rebased without conflicts (no shared CHANGELOG.md edit)
• Operator workflow: echo '### foo' > CHANGELOG.d/222-foo.md && git add CHANGELOG.d/ (one file, no conflict)

v1 follow-ups (out of scope):

• Apply the same pattern to AGENTS.md ("Last verified")
• Apply the same pattern to cmd/sin-code/main.go subcommand list
• CI gate that runs --check on every PR

Refs: #222

[vercel]

Deployment failed with the following error:

Resource is limited - try again in 24 hours (more than 100, code: "api-deployments-free-per-day").

Learn More: https://vercel.com/aragonheld-7756s-projects?upgradeToPro=build-rate-limit

[github-actions]

🏆 CEO Audit — A+ (100.0/100)

Metric	Value
Grade	A+
Score	100.0/100
Critical findings	0
High findings	0
Profile	QUICK
Min grade gate	B

📥 Download full report (Markdown)
📊 Download SARIF (for Code Scanning)

Run ~/.config/opencode/skills/ceo-audit/scripts/audit.sh . --profile=QUICK locally to reproduce.

[github-actions]

🏆 CEO Audit — A+ (100.0/100)

Metric	Value
Grade	A+
Score	100.0/100
Critical findings	0
High findings	0
Medium findings	0
Profile	QUICK
Min grade gate	B

📥 Download full report (Markdown)

Run ID: 27649462231 · Commit: ${github.sha}

Run ~/.config/opencode/skills/ceo-audit/scripts/audit.sh . --profile=QUICK locally to reproduce.