Add experimental wt step promote command

[max-sixty]

Summary

Adds wt step promote — an experimental command to exchange branches between the main worktree and another worktree. This addresses #738 where users want to move a branch from a worktree into the main worktree.

This is useful when the main worktree has special significance (Docker Compose configs, IDE settings, heavy build artifacts anchored to project root) and hooks aren't yet set up to run on arbitrary worktrees.

Key features:

• Exchange branches between main worktree and any other worktree
• Auto-restore: wt step promote from main worktree (no args) restores default branch
• Shows mismatch state in wt list with ⚑ indicator
• Warns when creating mismatch, with hint to restore

Example:

# from ~/project (main worktree)
$ wt step promote feature

Before:

  Branch   Path
@ main     ~/project
+ feature  ~/project.feature

After:

  Branch   Path
@ feature  ~/project
+ main     ~/project.feature

To restore: wt step promote main from anywhere, or just wt step promote from the main worktree.

Why experimental

The idiomatic Worktrunk workflow doesn't use promote — instead each worktree has a full environment. This command exists as a transitional tool while migrating to that model.

Test plan

• New test suite in tests/integration_tests/step_promote.rs
• Tests for: branch exchange, restore, auto-restore, dirty worktree errors, mismatch indicator in list
• All 923 tests pass

Closes #738

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This was written by Claude Code on behalf of @max-sixty

[max-sixty]

Limitation: gitignored files don't move with the branch

The current wt step promote only swaps branches — gitignored/untracked files (target/, node_modules/, .env) stay in their original filesystem locations. A developer who runs expensive builds in a linked worktree loses their build artifacts when promoting.

Why this is tricky:

• git worktree move doesn't work for the main worktree — Git doesn't support it. This is the same limitation relocate hits; for main worktree relocation, it checks out the default branch and creates a new linked worktree elsewhere (files don't move).

• Filesystem swap + metadata repair is risky — We could swap directories and fix .git/worktrees/*/gitdir pointers, but the main worktree has an embedded .git/ directory while linked worktrees have a .git file. Lock files, hooks, and index state could get confused. High risk of corruption.

• Copy-based swap has tradeoffs — Using the copy-ignored infrastructure (reflink where available) avoids touching git metadata, but: not atomic (power failure mid-swap loses files), needs 2x disk space during swap, slow for large directories even with reflink.

• Accepting the limitation — Document that promote only swaps branches. Simple and safe, but users must manually copy files or rebuild.

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This comment was written by Claude Code on behalf of @max-sixty