docs: comprehensive hook/sling/handoff/nudge audit

- Update propulsion-principle.md: implementation status now accurate
- Update beads-data-plane.md: correct command syntax
- Fix hook.go: clarify durability semantics, add related commands
- Fix sling.go: use reliable NudgePane instead of raw tmux send-keys
- Add tmux.NudgePane: pane-targeted reliable message delivery

The command menagerie:
  gt hook    = assign (durability)
  gt nudge   = communicate (generic messaging)
  gt sling   = hook + nudge "start working"
  gt handoff = hook + restart (GUPP kicks in)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/beads-data-plane.md

@@ -268,7 +268,9 @@ The **handoff mail is supplementary context** (optional but helpful).
 | `gt mail inbox` | Query: `assignee = me AND status = open AND from:*` |
 | `gt mail read X` | Read issue X, no status change |
 | `gt mail delete X` | Close issue X (status → closed) |
-| `gt sling X to Y` | Update X: `assignee = Y, pinned = true` |
+| `gt sling X [Y]` | Hook X to Y (or self), inject start prompt |
+| `gt hook X` | Update X: `pinned = true` (assign without action) |
+| `gt handoff X` | Hook X, restart session (GUPP kicks in) |
 | `bd pin X` | Update X: `pinned = true` |
 | `bd close X` | Update X: `status = closed` |