…tests

Follow-ups from the multi-agent review of PR #21 (merged). Three agents
converged on merge_session_start_hook under-delivering its docstring.

Behavior (review #1/#10):
- merge_session_start_hook now canonicalises Clarion-owned hooks to exactly
  one running the desired command: refresh the first, remove any extras (a
  stale duplicate, or one pinned to a different project), and drop a
  Clarion-dedicated group left empty. Previously it short-circuited on a
  current entry and refreshed only the first stale one, so a current hook
  coexisting with a wrong-project pin (hand-merged settings) silently
  oriented the wrong project every session, and multiple stale entries left
  survivors. Idempotency preserved (single exact match → no-op). Tests:
  different-path refresh, current+stale removal, multi-stale dedup.

Tests / docs:
- schema.rs: migration_0002_records_its_own_version_row exercises 0002's SQL
  directly (bypassing apply_one's INSERT OR IGNORE fallback) so the
  in-transaction version-row INSERT is load-bearing — verified it fails if
  that line is dropped. The rollback test now also asserts the version-2 row
  rolls back together with the column. (review #2)
- install.rs: correct the non-dir-guard comment — only --all reaches that
  block (--skills/--hooks alone are Components with init_clarion()==false).
  (review #3)
- install.rs tests: cover the --force non-directory .clarion guard. (review #7)
- hooks_settings.rs: shell_single_quote("") -> "''" edge case. (review #6)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>