perf(agent): skip session JSONL sanitize when file is unchanged

[arnohillen]

Problem

Part 2 of the task re-open latency work (see #3976). Every agent reconnect runs sanitizeSessionJsonl over the native Claude transcript (~/.claude/projects/.../<sessionId>.jsonl): a full read plus a JSON.parse per line. These files grow to tens of MB on long sessions, so every re-open of a cold task pays seconds of parse work before the session can resume, even when the file was already verified clean moments earlier.

Changes

• Remember the file stat (mtime + size) of the last clean or healed pass in a small bounded module cache (lru-cache, already a workspace dependency), and return early when the current stat matches.
• The Claude SDK only appends to the JSONL, which changes both size and mtime, so any real change forces a full re-parse. A stale memo can only cause extra work, never a skipped heal. The stat-equality assumption is the same one the existing concurrent-writer guard already relies on.

How did you test this?

• New test proving the skip through the public interface: a dirty file whose stat is restored to the memoized clean-pass stat must not be healed (would fail if the memo check were removed), and a file that grows past a clean pass must be re-sanitized.
• Full jsonl-hydration suite: 66 tests passing.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 65ea453.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "perf(agent): skip session JSONL sanitize..." | Re-trigger Greptile}

[charlesvien]

Hey @arnohillen, we've migrated this repo into the PostHog/posthog monorepo, so this PR needs to be recreated there.

Easiest path: check out the monorepo and run the /porting-code-prs skill with your agent pointed at this PR. It has the migration guide and path remappings baked in so the agent doesn't have to figure them out. You don't strictly need it though, just asking your agent to port the PR works fine too.

Shout if you get stuck.

[arnohillen]

@charlesvien ported to the monorepo as PostHog/posthog#76791, via the /porting-code-prs skill (original commits and authorship preserved with git am, lockfile change re-derived against the monorepo lockfile). Fine to close this one once the port merges.