feat(mobile): per-user device bindings, multi-user Inbox routing, and admin-mediated authorization

- Device→user bindings persisted in config table (auth.rs), loaded at plugin start
- RelayApp now routes Inbox responses per-user via UserChannelApi, never globally
- New mobile_bind_device LLM tool for admin-mediated device→user assignment
- Per-user event forwarders (events.rs) with per-user debounced notifiers
- Config listener (auth::config_listener) refreshes bindings cache reactively
- Reconcile loop catches users who unlock after boot
- Hello/Logout treated as device-registry ops (no user resolution needed)
- Unbound device payloads are silently dropped
- RelayAgent::authorize_client → bind_device (atomic bind + authorize)
- Approval rules seed mobile_bind_device/revoke_device as require

crates/skald-core/src/approval/mod.rs

@@ -235,6 +235,10 @@ impl ApprovalManager {
             // Opening a mobile pairing window emits a secret (the QR) into chat:
             // it must be a deliberate human action, not LLM-triggerable (plugin.md §11).
             ("mobile_start_pairing", "require"),
+            // Binding/revoking a device assigns a phone to a user — a security
+            // decision (who receives whose Inbox), so it must be human-gated too.
+            ("mobile_bind_device", "require"),
+            ("mobile_revoke_device", "require"),
         ];
         // NOTE: file-write tools are NOT seeded here as per-tool `require` rules.
         // Filesystem gating is owned by the "File System" category — path-scoped