Heal torn base tails on write and keep failed rewrite commits on HEAD

[TheGreatAxios]

Summary

• writeTurns no longer crashes on a torn or poisoned turns.jsonl base tail — it falls back to the same resilient on-disk parse load() uses, so the write that heals the file can actually run
• A failed compact commit now restores the working-tree turn segments to the published generation instead of leaving the uncommitted rewrite on disk for the next load() to serve
• The commit-failure test now forces a real rejection under isomorphic-git (which never runs .git/hooks/*) by replacing .git/objects with a regular file — the previous commit-msg hook mechanism could never fail the commit it claimed to exercise
• assemble-runtime tests mock createSessionStores — the boundary buildAgent actually calls since the store-factory split — instead of the inner createOptimizedContextStore that no longer runs there

Test plan

• bun test src/session/optimized-context-store.test.ts src/session/assemble-runtime.test.ts — 40 pass
• bun test ./src/session — 206 pass
• bun run typecheck — clean

[linear-code]

CL-7700