Extract restart operation to core/operations/restart.ts

[2witstudios]

Epic 1: Core Operations Refactoring — Issue 2/4

Source: 1-agent worktree (wt-a6gki1)

Description

Move agent restart logic from commands/restart.ts into core/operations/restart.ts. CLI command becomes a thin wrapper.

Acceptance Criteria

• core/operations/restart.ts exports a performRestart() function
• Handles tmux window creation for restarted agent
• Updates manifest: new agent entry, old agent status → gone
• commands/restart.ts simplified to arg parsing + calling performRestart()
• Tests: tmux window creation, manifest update, old agent cleanup

Files to Create/Modify

• src/core/operations/restart.ts (new)
• src/core/operations/restart.test.ts (new)
• src/commands/restart.ts (simplify)