fix: block uncertain writes across checkpoint resume paths

[TaewoooPark]

Fixes #7.

An interrupted write was classified as read-only, and interactive /resume / --continue could load its pending checkpoint without the uncertainty check. This can continue a conversation without knowing whether the write took effect.

This patch records new writes as mutating and checks both the stored flag and current classification, covering legacy journals with mutating: false. Core, journal and interactive continuation refuse uncertain execution before installing history or calling the model. A failed --continue also does not silently submit its initial task as a new run. Read-only and completed interactive conversations remain resumable; existing compatibility checks and journal format are unchanged.

Validation:

• 24 added regressions: lifecycle interruption boundaries, legacy/current flags, stored true flags, corruption/compatibility precedence, and all continuation entry points. The relevant refusal cases fail on the original implementation.
• Built CLI against a localhost fixture: legacy false write goes from exit 0 / one model request to exit 2 / zero requests; a durable completion checkpoint still resumes. This proves unsafe acceptance was blocked, not that the baseline repeated the write.
• Actual macOS PTY: /resume and --continue × legacy false / true write / read-only / completed, all 8 cases pass. Refusal leaves history/channel intact and sends zero model requests.
• pnpm test: 726 tests across 42 files; pnpm typecheck, pnpm build, pnpm lint:tools, and git diff --check pass locally. GitHub CI passes all five jobs: Node 20/22, Linux ARM64, Python, and template goldens.

Review window: until 2026-09-25 00:30 KST (2026-09-24 15:30 UTC). Please review this PR over the next two days, especially legacy journal compatibility and interactive continuation. It will remain open for review; no merge or release is part of this change.

[TaewoooPark]

The previously announced review window has not quite ended, but there are no outstanding change requests, so I am wrapping up the review and merging this now.

The head remains 502bb67. The maintainer checks covered legacy/current uncertain writes, read-only and durably completed resumptions, and the interactive continuation paths. The combined build with current main and #34 passed 770 tests plus the actual CLI/PTY fixture checks; all five checks on this PR are green: https://github.com/TaewoooPark/Motifcode/actions/runs/35745240399.

This will close #7 and is planned for 0.3.4. Thanks to everyone following the session-recovery work!