fix(#147): repair torn trailing line so guard reload can't brick claude --resume

[junaidtitan]

Fixes #147 — the highest-priority data-loss class.

Root cause

A guard terminate-first reload SIGTERMs Claude Code. If Claude was mid-append, it leaves a torn/partial trailing JSON line. The deferred writer then snapshot-conflicts (file changed) and — per the #106 invariant — skips the write, leaving Claude's file in place. Nothing repaired the torn line, so claude --resume could no longer parse the transcript: "Failed to resume session." cozempic's prune logic is sound (validation aborts a bad prune); it's the terminate that leaves the torn line.

Fix

• session.repair_torn_trailing_line() — atomically drops a single torn trailing line (data the writer never finished → already unrecoverable; dropping it strictly improves resumability). Conservative: no-op if the last line is valid JSON; won't blank a file whose only line is torn; keeps a .torn.bak; never raises.
• Guard calls it on the deferred-write conflict/oserror branches (the cases that leave Claude's file), before the resume it triggers.
• Doctor new unresumable-session check + --fix so anyone who already hit this recovers in one command (cozempic doctor --fix).

Safety

• Only ever drops a provably-unparseable trailing line — never a valid line, never violates Guard daemon rewrites the live transcript out from under Claude Code (data-loss race) #106.
• Fully exception-isolated in the guard's post-terminate cycle.
• Fleet-reviewed (5 dimensions, adversarial verify): data-safety / guard-cycle-safety / edge-cases found nothing; 2 low follow-ups (OSError-branch test + receipt-file 0o600 hardening) applied.

12 new tests (helper edge cases, doctor check/fix, end-to-end guard conflict and oserror paths; mutation-verified). Full suite 1757 passing.

Follow-up (separate, tracked): cozempic uninstall to undo init (also requested in #147).

[Copilot]

Pull request overview

This PR addresses #147 by making Claude Code sessions resilient to a terminate-first guard reload that can leave a torn (partial) trailing JSONL line, which breaks claude --resume. It adds a targeted repair helper, wires it into guard/doctor flows, and hardens receipt file permissions.

Changes:

• Add session.repair_torn_trailing_line() to drop a single unparseable trailing JSONL line (best-effort, atomic, backup kept).
• Invoke the repair on guard deferred-write skip/failure paths and add a doctor check/fix for already-corrupted sessions.
• Harden receipts storage permissions and add tests covering the new behaviors.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_resume_torn_line.py	New tests for torn-line repair helper, doctor check/fix, and guard deferred-write skip paths.
tests/test_receipts.py	Adds assertions that receipts dir and files are user-only permissioned.
src/cozempic/session.py	Introduces repair_torn_trailing_line() to safely drop a torn trailing JSONL line.
src/cozempic/receipts.py	Tightens receipt file mode and receipts directory permissions.
src/cozempic/guard.py	Calls torn-line repair after deferred prune write is skipped or fails, before triggering resume.
src/cozempic/doctor.py	Adds unresumable-session check and --fix repair for torn trailing lines.

Comments suppressed due to low confidence (1)

src/cozempic/receipts.py:89

• os.open(..., mode=0o600) only applies the mode when the file is newly created; if a receipt file already exists with broader permissions (e.g. from an older version), appends will keep it world/group-readable. Since this change is explicitly aiming for user-only receipts, explicitly chmod/fchmod the fd best-effort after opening so existing files are also hardened.

    # 0o600: receipts are de-identified but still local provenance — keep them
    # user-only so another local user on a shared host can't read them.
    fd = os.open(path, os.O_WRONLY | os.O_APPEND | os.O_CREAT, 0o600)
    try:
        os.write(fd, data)
    finally:
        os.close(fd)

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