docs(claude): rewrite release section for auto-promote workflow + link-ref on promote (#437)

Two paired updates:

1. **`CLAUDE.md` § Releases** — rewritten to match the actual workflow now
   that #436's auto-promote step lands the entries automatically.

   The old text told Claude to 'Add a new `## [X.Y.Z] - YYYY-MM-DD`
   block at the top of CHANGELOG.md' as the first step. That instruction
   is the exact pattern that caused the v0.9.5 sparse-release-notes
   incident — a hand-added sparse `[X.Y.Z]` block (one early fix
   pre-staged) is what the extractor picked, ignoring everything under
   `[Unreleased]` above it.

   New default: write entries under `## [Unreleased]` during normal
   work. The Release workflow promotes them at release time. The
   formatting rules (sub-section grouping, user-perspective wording,
   issue/PR refs) are preserved. The link-reference rule moves to 'don't
   add it yourself' since `prepare-release.mjs` now appends it.

2. **`scripts/prepare-release.mjs`** — extended to also append a
   `[X.Y.Z]: https://github.com/colbymchenry/codegraph/releases/tag/vX.Y.Z`
   link reference at the end of CHANGELOG.md when promoting (idempotent
   — no-op if one already exists, regardless of where in the file it
   sits). This is what makes the `## [X.Y.Z]` heading text auto-link
   to its release tag in GitHub's renderer; without it the heading still
   renders, just unlinked. 3 new tests cover Case A append, Case B
   append-when-merging, and no-double-add.

940/942 existing tests still pass (2 pre-existing skips); +3 new tests.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>