Add typeorm-migration agent skill

[TaprootFreak]

What

Adds a typeorm-migration agent skill at skills/typeorm-migration/ (SKILL.md + reference.md) that
documents how to author a database migration in this repo: generate from the entity diff, hand-write
schema migrations with TypeORM's deterministic constraint naming, and follow the post-merge
immutability rule.

Why this is needed

Writing a migration here carries non-obvious, load-bearing knowledge that gets re-derived every time
and is partly CI-enforced:

• Deterministic constraint naming — hand-written schema migrations must reproduce
  <prefix>_ + sha1(table + '_' + sortedColumns).substring(0, N) with the exact per-prefix N
  (27 for PK/FK/UQ/DF, 26 for REL/IDX/CHK). Get it wrong and the next generated diff fights the
  migration. CONTRIBUTING.md memorializes this table precisely because it is easy to get wrong.
• Immutability — .github/workflows/api-migration-check.yaml blocks any modify/delete/rename of
  an existing migration; the rule is "add a follow-up", not "edit". A skill turns this into a
  runnable procedure instead of tribal knowledge.
• Generation prerequisites — migration/dev-data-source.ts is git-ignored and must be created
  locally, and the build + DB must be current or the diff is wrong. New contributors hit this.

This is a per-PR, knowledge-heavy procedure backed by an existing command (npm run migration) — the
same "wrap an existing CLI with curated knowledge" shape as the db-debug skill (#3957). The
skills/ convention and its CONTRIBUTING section are introduced by #3957; this PR only adds the skill
directory and does not touch CONTRIBUTING.

Files

• skills/typeorm-migration/SKILL.md
• skills/typeorm-migration/reference.md

PR completeness

• Migration: N/A (no entity/column changes — docs only)
• Environment / Infrastructure: N/A
• Service updates: N/A
• Frontend synchronization: N/A

Related: #3957 (db-debug skill — establishes the skills/ convention).