Rollback on partial install failure

[dean0x]

Problem

If installViaFileCopy crashes mid-plugin (e.g., disk full, permission error), partial artifacts remain in ~/.claude/commands/devflow/, ~/.claude/agents/devflow/, and ~/.claude/skills/. No rollback mechanism exists.

Proposed Fix

1. Copy to a temp directory first, validate all files are present
2. Move atomically to final location (rename is atomic on same filesystem)
3. Track installed files in manifest for clean rollback
4. On failure: remove partial artifacts and restore previous state

Scope

• Medium effort — temp directory staging, atomic move, rollback logic
• Improves reliability for users with constrained environments

[dean0x]

Closing: Install is idempotent — just re-run on failure. Partial install failures are extremely rare in practice.