refactor: Add dependency injection for gitCommand, tmux, and Bun.spawn

[nigel-dev]

Source

Audit report — Section 7: Technical Debt > Architecture

Description

orchestrator.ts, worktree.ts, integration.ts, and other core modules directly import and call gitCommand(), tmux functions, and Bun.spawn(). This makes unit testing difficult — tests must mock module-level imports via spyOn rather than injecting test doubles through constructors.

Current State

• orchestrator.ts directly imports from tmux.ts, git.ts, integration.ts, worktree.ts
• Tests use spyOn(module, 'function') pattern extensively (fragile, order-dependent)
• No interface abstractions for git operations, tmux, or process spawning

Proposed Solution

1. Define interfaces: GitClient, Multiplexer, ProcessRunner
2. Accept these via constructor injection in Orchestrator, MergeTrain, WorktreeProvider
3. Provide default implementations wrapping current functions
4. Tests inject mocks directly — no more module-level spying

Relationship to Other Issues

• Prerequisite for feat: Abstract tmux behind a Multiplexer interface #27 (Multiplexer interface) — DI makes swapping tmux for Zellij natural
• Aids refactor: Decompose Orchestrator god object into focused modules #17 (Orchestrator decomposition) — extracted modules take dependencies via constructor
• Improves test: Add test coverage for untested core files #25 (test coverage) — makes previously hard-to-test code testable

Priority

P3 — architectural improvement, not blocking any functionality. Best done alongside #17 decomposition.