feat: implement gt sling and wisp hook system (gt-qvn7.1)

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

.gitignore

@@ -29,3 +29,6 @@ gt
 # Runtime state
 state.json
 .runtime/
+
+# Ephemeral wisps (never tracked)
+.beads-wisp/