V-L2-F4: harden JSON sidecar (cross-process locking + atomic writes) and wire `provenance`/`history` CLI

[hyperpolymath]

Follow-up to #146 (JSON-family sidecar backend). Two threads:

1. Harden the JSON store

• Cross-process write locking: serialise concurrent writers to a json sidecar (the SQLite path serialises via the DB write lock; the json path was single-writer-by-assumption). Dependency-free advisory lock file (<path>.lock) with backoff + stale-lock steal; RAII release.
• Atomic writes: formalise the temp-file + same-dir rename durability already in save(), held under the write lock across the load→mutate→save cycle.

2. Wire the stub CLI subcommands

• provenance <entity>: list the entity's provenance chain and report verification (verify_chain) + any fork points. Both sqlite and json backends.
• history <entity> [--at T]: list temporal versions (or the point-in-time snapshot at T) for the entity. Both backends.
• postgres reads remain explicit "not yet implemented".

3. Docs + config

• rustdoc + README section on sidecar backends and the now-working subcommands.
• Example manifest showcasing storage = "json" / format.
• Version bump + CHANGELOG.
• .claude/settings.json permission allowlist to reduce harness friction.

Acceptance

• JsonStore writes are guarded by a cross-process lock and remain crash-safe (atomic rename); concurrent gc/append don't corrupt or lose data.
• provenance and history work against both sqlite and json sidecars (tests cover both).
• Docs, example, version/CHANGELOG, and settings updated.
• CI (fmt/clippy/test on MSRV+stable) green at source.

[hyperpolymath]

Closed by #151 (00fd80f). Delivered:

• Cross-process write locking — dependency-free advisory <path>.lock (O_EXCL, retry/backoff, stale-steal, RAII release) via json::with_locked, held across load→mutate→save; gc apply runs through it. Atomic temp-file + rename durability documented.
• provenance / history CLI wired for sqlite + json (postgres refuses); end-to-end test (tests/sidecar_cli_test.rs) seeds via the locked write path and drives the binary.
• Docs: README backend section + per-command matrix, rustdoc, examples/json-sidecar/, version 0.1.0→0.2.0 + CHANGELOG.

One item not completed: the .claude/settings.json permission allowlist was blocked by the agent self-modification guardrail (an agent can't widen its own permissions). It needs to be added by a maintainer directly if desired — everything else in the acceptance is done.

---

Generated by Claude Code

[hyperpolymath]

Decision record added: ADR-0013 (docs/decisions/0013-json-sidecar-cross-process-locking.adoc) now documents this hardening — the cross-process advisory lock, atomic-rename durability, and the wired provenance/history CLI — framed as refining ADR-0012 (the F3 JSON-family backend). ADR-0012's single-writer note was updated to point at it, and a CHANGELOG.md entry added (#153). No reopen needed — #150 stays closed/completed (implemented in #151).

---

Generated by Claude Code