What

Adds e2e/integrity.test.ts (@p0, fully offline): a data-integrity tier that proves store guarantees the docs claim but no existing tier verifies. 7 tests, ~6s locally.

Why

The single-writer serialization guarantee, path-traversal rejection, and no-op idempotence were asserted in docs but unproven end-to-end. Each test fails if the behavior regresses.

Tests

1. 10 concurrent writes serialize into 10 clean commits - starts a managed daemon on an ephemeral port, fires 10 parallel memory write to distinct paths; asserts all exit 0, all readable, exactly 10 commits, clean working tree, no index.lock, git fsck clean.
2. 5 concurrent writes to one path stay consistent - 5 parallel writes to the same path; final content is exactly one of the 5 bodies (no torn blend), 5 commits, clean tree, fsck clean.
3. Path traversal + absolute paths refused - ../escape.md, @<vault>/../escape.md, and an absolute path are all rejected with a canonical invalid path error; the escaped file is never created.
4. No-op write adds no commit - writing identical content twice: second write exits 0, mints no new commit, and the reported rev equals current HEAD (regression guard for the byte-identical write path).
5. stdin body round-trips - echo body | memory write ... --body - writes and reads back.
6. Offline update check degrades gracefully - endpoint pointed at an unresolvable host; update --check exits 0 with a calm verdict, no hang (hard timeout), no stack trace. Behavior-based so it survives the in-flight update-check rework.
7. vault remove is non-destructive - files, .git, and full commit history stay on disk; only the registry entry is removed.

Notes

• Follows existing harness patterns (createCliMachine, freePort, isolated AGENTAGE_CONFIG_DIR, ephemeral ports, NO_COLOR). Local-only helpers live in the spec; helpers.ts untouched (sibling PRs in flight).
• Concurrency + serialization are exercised through the managed daemon (single shared engine); traversal/no-op/stdin/remove use the in-process engine with egress blackholed.
• Verified: spec 3x consecutively clean; full offline e2e set green together (no interference); npm run verify green.