feat(decision-records): flatten accepted/ into one directory

[trungfinity]

What

S1 of the decision-records convention refresh: accepted/<type>/ collapses to a flat accepted/ — the type subdirectory was a layer of hierarchy the convention never needed, since identity is the global counter, type already lives in front-matter, and the INDEX groups by it. All three lifecycle dirs are now flat.

How it lands

• promote writes to accepted/NNNN-title.md; drafts/ and accepted/ being siblings means same-depth relative links survive promotion unchanged, and a path into the old drafts/ dir is still re-expressed.
• The new-type heads-up now reads the set of types the accepted records carry (front-matter) instead of listing directories.
• check refuses a nested record with the fix by name: the new one-shot migrate-layout command. It refuses duplicate filenames before moving anything, re-paths each moved record's hand-authored relative links one level up, retargets links elsewhere in docs whose target record moved (the class neither --relink nor spelled-out-path rewriting reaches), repoints spelled-out paths, and finishes with build --relink.
• Filenames — the identity — survive the move, so the origin/main collision warning stays correctly silent across a migration, even while origin/main is still nested mid-migration.

Consumers

No flag-day: consumers pin by hash in skills-lock.json and migrate when they bump. lanh-ai (55 records in 4 type dirs) is the only real migration; axie-principles has zero accepted records. Follow-up PRs handle each.

Verification

• 105 tests pass, including 5 new migrate-layout tests (flatten + relink + retarget, check guidance, no-op idempotency, refuse-before-move on duplicates).
• Smoke-tested migrate-layout against a copy of lanh-ai's real docs tree: all 55 records flattened, 70 files relinked, zero residual link breakage (the only check complaints in the sandbox were files outside the copied tree).
• validate_skills.py and pre-commit run --all-files clean.