feat(dev): write plan files as the run goes

[tobrun]

Summary

• scope, scope-review, build and ship now write their `.dev/{plan-name}/` file as the run goes (spec during the interview, review files before the panel returns, build run and fixup entries, `pr.md` filled check by check) instead of at the end of a stage
• `check-tests.py` ends a change set's notes entry at any other `##` heading, so a fixup's `Tests added:` no longer counts toward the change set above it
• Records D-written-as-the-run-goes, D-ship-record-in-pr-body, D-notes-fixup-entry; bumps dev to 3.5.0 (Codex plugin regenerated)

Test plan

• `scripts/validate.sh` passes
• `python3 scripts/build_codex_plugin.py --check` is up to date
• new unit test for the fixup heading in `test_build_speed.py`

🤖 Generated with Claude Code