docs: correct the v2.1.0-beta / 2.2.0-beta split in CHANGELOG, BREAKING, migration.md

[ocots]

Summary

v2.1.0-beta was tagged 2026-07-30 (ct-registry) and is not in General. Every PR since
(#830 through #893) landed in 2.2.0-beta instead — but three published files still
attribute post-tag behaviour to v2.1.0-beta.

Found while auditing the documentation campaign's own reports for the same mix-up
(they said "Target: v2.1.0-beta" and were corrected first).

What's wrong today

• src/deprecated.jl (the five v2.0 → v2.1 compatibility shims, PR feat(deprecation): v2.0 compatibility shims for v2.1.0-beta #855) was added
  2026-08-15, two weeks after the v2.1.0-beta tag. It is in neither CHANGELOG entry.
  docs/src/migration.md reads as if every removed spelling already gets a named
  PreconditionError "in v2.1.0-beta" — false for that exact tag, which still throws a
  bare UndefVarError/MethodError.
• Reject unconsumed options in explicit solve mode #853 (explicit solve now rejects unconsumed keyword arguments instead of
  silently ignoring them — near-breaking) is in neither CHANGELOG.md nor BREAKING.md.
• BREAKING.md has three # H1 sections and no index.
• CHANGELOG.md's [2.2.0-beta] entry doesn't follow the Handbook's emoji section
  taxonomy (philosophy/changelog.md), unlike CTParser 0.9.0 and CTDirect's changelog.

What changed

• CHANGELOG.md — [2.2.0-beta] entry rewritten under the emoji taxonomy (✨ 🐛 🛠
  🔄 🧪 📚 📦 ✅). New ✨ entry for the shims, explicit that they postdate the
  v2.1.0-beta tag. New 🛠 entry for Reject unconsumed options in explicit solve mode #853. New 🔄 line for the docs-build label gate
  (ci(docs): build the documentation on documentation-relevant pull requests #875 reverted by ci(docs): restore the 'run documentation' label gate #876). ✅ footer now names two near-breaking notes. Added the
  <!-- markdownlint-disable MD024 --> header the Handbook template specifies.
  Scope: only [2.2.0-beta] converted; older entries untouched.
• BREAKING.md — single H1 with a three-line index; the two epoch sections and
  everything nested under them (34 headings) demoted one level. Anchors are text-based
  in GitHub markdown, so the one external reference
  (docs/src/migration.md → #breaking-changes-v1x--v20) still resolves — checked.
  Added the Reject unconsumed options in explicit solve mode #853 near-breaking note.
• docs/src/migration.md — a warning box clarifying that the informative shim
  messages need 2.2.0-beta, not just the v2.1.0-beta tag. Gave "What you get instead
  of an error" an explicit @id so the box can link to it. The other ten
  v2.1.0-beta mentions on the page were read individually and left as-is — each
  correctly describes when a spelling stopped working, not the shim behaviour.

Verification

grep -c "^## \[" CHANGELOG.md      → unchanged (entry count preserved)
grep -n "^# " BREAKING.md          → 1 (was 3)
grep -rn "BREAKING.md#" . --include=*.md   → the one external link, anchor text unchanged

No source file touched — documentation coherence only, no test run needed.

🤖 Generated with Claude Code