Spec Kitty

Mission Run: Mission-identity naming seam & #1908 panel hardening

Establish one canonical authority for mission-identity branch/worktree naming (fixing a recurring mid8 regression class incl. a P1 merge-blocker) and close the remaining post-#1908 adversarial-panel robustness findings.

The slug-to-mid8 branch/worktree naming is reimplemented ad-hoc in several places, each with its own idempotency or parsing bug — including #1978 which blocks spec-kitty merge for missions whose slug embeds its mid8. This mission binds that mapping to a single canonical seam and routes the broken call sites through it, then clears the remaining post-#1908 adversarial-panel findings (lane-merge atomicity, base-ref argument hygiene, and an accept-gate identity side-effect) so the merge/lane/accept paths are robust.