Add Migrator director for reversible data migrations

[TheGreatAxios]

Summary

• Adds a Migrator leaf director that plans reversible settings, config, and session-state data changes with forward migration, rollback, dry-run evidence, and in-flight session impact
• Registers Migrator in the closed director set and the Skywalker spawn allowlist

Verification

• bun run typecheck passes
• bun run check passes except one flaky timing test in src/tui/tool-execution-watchdog.test.ts that passes in isolation
• bun test ./src/agent/directors/ ./src/agent/prompt-sizes.test.ts ./src/agent/directors-prompt.test.ts passes (307 pass, 0 fail)

Fixes CL-7671

[linear-code]

CL-7671

[TheGreatAxios]

Audit (Migrator director): Approve contingent on rename nit.

Findings:

• Budgets + routing present.
• Docs: none on this PR — after Add a Gauntlet director that mutation-checks tests can actually fail #977 lands as the template, consider a small docs follow-up matching its shape if the team wants parity.
• Naming deviation (required fix): migrator/package.ts systemPrompt opens with You are Migrator, but the convention across siblings is You are XDirector (X) (Gauntlet: You are GauntletDirector (Gauntlet), Prober: You are ProberDirector (Prober), Warden: You are WardenDirector (Warden)). Please rename to You are MigratorDirector (Migrator) and update any test pins asserting the old wording. A minimal fix commit will be pushed on this branch.
• Collision: same 16->17 bump as Add a Gauntlet director that mutation-checks tests can actually fail #977/Add measure-only prober leaf director #962/Add warden trust-review director for permission and plugin paths #966 — rebase to 18/19/20 after Add a Gauntlet director that mutation-checks tests can actually fail #977 lands, during merge phase (not stacked).