Code review fixes:
1. CRITICAL: Move polecat check to start of runSling
- Previously wrote wisp THEN failed, leaving orphan
- Now fails fast before any file operations
2. CRITICAL: Sanitize slashes in agent IDs for filenames
- Agent IDs like 'gastown/crew/joe' were creating subdirs
- Now converts '/' to '--' for safe filenames
- Added sanitizeAgentID/unsanitizeAgentID helpers
3. MODERATE: Use git root instead of WorkDir in prime.go
- Hooks are written to clone root, not cwd
- Added getGitRoot() helper for consistency
4. MODERATE: Fix silent error swallowing
- Now logs non-ErrNoHook errors when reading hooks
- Warns if bead doesn't exist before burning hook
- Preserves hook if bead is missing for debugging
Phase 1 of tracer bullet: Slinging Handoff
- Add internal/wisp package for ephemeral work attachment
- Add gt sling command to attach work and restart
- Update gt prime to check/burn slung work on hook
- Add .beads-wisp/ to gitignore
