feat(ensign): add commit-before-idle and anti-idle rules

[iamcxa]

Ensigns that go idle without committing require FO nudges — up to 3 per session observed in practice. This PR adds two explicit rules to ensign-shared-core to prevent this failure mode.

What changed

• Add 'MUST commit before signaling completion' rule — prevents uncommitted-then-idle pattern
• Add 'Do not idle between steps' rule — prevents mid-task stall waiting for external input

Review guidance

These rules are derived from operational experience: the failure mode was 'ensign signals done, FO checks entity file, no commit, must re-dispatch.' The rules make the commit requirement explicit rather than implied by the Completion section.

[clkao]

@iamcxa Thanks! This PR contains other changes, can you split them and update the PR with clean FO/Ensign rules?

[iamcxa]

Done — rebased onto clkao/main HEAD (70575809), branch now contains only the ensign rules as requested:

• 36620b8b feat(ensign): add commit-before-idle and anti-idle rules to ensign-shared-core

1 commit, 1 file touched (skills/ensign/references/ensign-shared-core.md, +2 lines). Infrastructure commits (prompt-upgrade, skill-binding, workflow-adopt/sync renames, VCS detection) dropped — they're in their own PRs (#119, #120, #130, #134). Ready for re-review.