docs: refresh for v6.12.0

[tgorka]

Incremental, not a rewrite — the docs were rebuilt one release ago and
are structurally correct. Changed what v6.12.0 makes false.

• Pinned versions and counts everywhere they appear: core v6.12.0, bmb
  v2.2.2, cis v0.3.2, gds v0.7.2, tea v1.26.0, bmad-loop v0.11.1;
  110 -> 111 skills (bmm 35 -> 36); 20 -> 21 deprecated-described skills;
  a plain /bmad:init now creates 36, not 37. Every number derived from
  the tree or from a real init run, not carried over.
• The shim rationale is rewritten, not restated. v6.11.0.0 justified
  shipping the shims by quoting upstream's v6-shims/README.md. v6.12.0
  made them opt-in and left that README stale, so the same conclusion now
  rests on different evidence: GDS v0.7.2's live instructions and
  customize.toml values. README, AGENTS, the plugin README, the ecosystem
  audit and module-integration all say so in those terms, and
  module-integration gains an edge-case section — 'a module can depend on
  a skill the installer stops installing by default' — next to its
  existing sibling about the installer silently dropping skills.
• README gains a 'New in v6.12.0' block: bmad-walkthrough replacing
  bmad-checkpoint-preview, Build sizing its own ceremony, review triage
  logging a verdict per finding, bmad-project-context adopting a
  handwritten AGENTS.md, and the two renames that break custom overrides
  (persistent_facts now empty, {diff_output} -> {diff_file}).
• Ecosystem re-audited against gh api: bmad-plugins, bmad-eval-quality
  and cis-skills added with one line each on why none is a bundle
  candidate; bmad-skills recorded as no longer listed in the org.
• manual-testing gains a check for the dependency that justifies
  --shims, since that is what will one day retire it.
• CHANGELOG: 6.12.0.0 section, and the release-script fixes that were
  sitting under Unreleased are folded into it.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

---

Summary by cubic

Refreshes the docs for the v6.12.0 upstream release, updating every version, count, and the rationale for shipping the v6 deprecation shims.

• Pins core v6.12.0, bmb v2.2.2, cis v0.3.2, gds v0.7.2, tea v1.26.0, and bmad-loop v0.11.1 in all version tables.
• Updates skill counts: 110 → 111, deprecated shims 20 → 21, and /bmad:init now creates 36 files instead of 37.
• Rewrites the shim justification: upstream's v6-shims/README.md is stale, so the docs now cite GDS v0.7.2's live instructions and customize.toml references as the reason the sync passes --shims.
• Adds a "New in v6.12.0" block covering the bmad-walkthrough rename, build ceremony changes, review triage verdicts, and two override-breaking renames (persistent_facts now empty, {diff_output} → {diff_file}).
• Re-audits the ecosystem against gh api and adds bmad-plugins, bmad-eval-quality, and cis-skills with why each is not a bundle candidate; records bmad-skills as no longer listed in the org.
• Adds a manual-testing check for the GDS dependency that justifies --shims.
• Folds the release-script fixes under Unreleased into a new 6.12.0.0 CHANGELOG section.

^{Written for commit a58fd15. Summary will update on new commits.}