Extract spawn operation to core/operations/spawn.ts

[2witstudios]

Epic 1: Core Operations Refactoring — Issue 4/4

Source: 1-agent worktree (wt-a6gki1)

Description

Move worktree creation + agent spawning from commands/spawn.ts into core/operations/spawn.ts. This is the most complex extraction — handles git worktree add, env setup, tmux, prompt files.

Acceptance Criteria

• core/operations/spawn.ts exports a performSpawn() function with typed options
• Handles full spawn pipeline: generate IDs → git worktree add → setup env → create tmux window → split panes → write prompt → send-keys → update manifest
• Template variable rendering
• Returns worktree + agent IDs + tmux targets
• commands/spawn.ts simplified to arg parsing + calling performSpawn()
• Tests: full spawn pipeline with mocked execa

Files to Create/Modify

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