🤖 refactor: extract WorktreeManager for Devcontainer runtime prep

[ethanndickson]

Summary

Phase 1 prep for adding Devcontainer runtime support. This is a pure refactor with no behavior changes—extracting reusable worktree logic so it can be shared between WorktreeRuntime and the upcoming DevcontainerRuntime.

Context

The Devcontainer runtime will need to:

1. Create git worktrees on the host (same as WorktreeRuntime)
2. Build/start a devcontainer from the worktree
3. Execute commands inside the container (different from WorktreeRuntime)

To enable this, we need the worktree creation logic to be reusable. This PR extracts it into a standalone WorktreeManager class.

Changes

WorktreeManager extraction:

• New src/node/worktree/WorktreeManager.ts containing all worktree operations:
  • getWorkspacePath - compute workspace path from project + branch
  • createWorkspace - create git worktree with branch
  • renameWorkspace - move worktree and rename branch
  • deleteWorkspace - remove worktree and optionally delete branch
  • forkWorkspace - create new worktree from existing branch

WorktreeRuntime simplification:

• Now a thin delegator that composes WorktreeManager
• Retains initWorkspace (runs .mux/init hook) which is runtime-specific

Flag rename:

• deferredHost → deferredRuntimeAccess for clarity (runtime access is deferred until postCreateSetup completes, e.g., Coder workspace startup)

DevcontainerRuntime shell:

• Added as dead code (not wired into runtimeFactory)
• Composes WorktreeManager for worktree operations
• Stub initWorkspace - will build/start container in Phase 2
• Sets deferredRuntimeAccess: true (container not available until built)

Test reorganization:

• WorktreeManager.test.ts - tests worktree logic directly (tilde expansion, delete behaviors)
• LocalBaseRuntime.test.ts - tests resolvePath with minimal test subclass
• Removed WorktreeRuntime.test.ts (now a thin delegator, logic tested via WorktreeManager)

Verification

• make static-check passes
• All existing tests pass
• No behavior changes (WorktreeRuntime delegates to same logic)

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