Add build-cache hygiene with dry-run cleanup and retention policy for app-managed worktrees

[baron]

Impact

App-managed worktrees can leave behind large .build directories after branches are abandoned, merged, or no longer active. Developers can reclaim space manually, but ad-hoc deletion is risky: it is easy to remove build state from a dirty or actively used worktree.

The build-storm mitigation work adds build-cache visibility. The next step is a safe, explicit hygiene flow.

Current behavior

• Build-cache size/footprint can be inspected.
• There is no conductor-managed cleanup plan.
• There is no retention policy.
• There is no dry-run eligibility report explaining what is safe to remove.
• Manual cleanup risks touching active, dirty, or in-progress worktrees.

Expected behavior

RepoPrompt CE should provide a safe build-cache hygiene flow for app-managed worktrees.

The default mode should be non-mutating and explain:

• which .build directories exist
• how large they are
• whether they appear stale
• whether they are eligible for cleanup
• why a cache is skipped

Actual deletion should require explicit confirmation and should only delete build-cache directories, never source trees or git state.

Proposed scope

Add a conductor-native cleanup planning flow, likely as an extension of build-cache diagnostics.

It should classify worktree build caches using signals such as:

• dirty vs. clean worktree state
• active daemon jobs
• live/bound agent sessions where available
• branch/worktree staleness
• age or last build activity
• cache size

Safety rules should be conservative:

• skip dirty worktrees
• skip worktrees with active jobs
• skip live/bound worktrees
• never delete worktree source directories
• require explicit confirmation for mutation
• default to dry-run

Non-goals

• Deleting git worktrees themselves.
• Automatic scheduled cleanup without user confirmation.
• Shared SwiftPM scratch/build storage strategy.
• Target modularization; see Modularize build targets so focused tests do not rebuild the monolithic RepoPrompt app target #416.

Acceptance criteria

• A dry-run command reports build-cache candidates with size, eligibility, and skip reason.
• Dirty worktrees are never eligible for cleanup.
• Worktrees with active daemon jobs are never eligible for cleanup.
• Live or bound worktrees are skipped when that state is detectable.
• Actual deletion requires explicit confirmation.
• Cleanup removes only build-cache directories, not source trees or git metadata.
• Post-clean output reports reclaimed space and skipped entries.