Extract kill operation to core/operations/kill.ts

[2witstudios]

Epic 1: Core Operations Refactoring — Issue 3/4

Source: 1-agent worktree (wt-a6gki1)

Description

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

Acceptance Criteria

• core/operations/kill.ts exports a performKill() function
• Handles tmux pane kill for individual agents
• Handles multi-agent worktree kill (kill all agents in worktree)
• Updates manifest: agent status → killed
• commands/kill.ts simplified to arg parsing + calling performKill()
• Tests: tmux pane kill, manifest update, multi-agent scenarios

Files to Create/Modify

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