feat(bridge): establish local pi status bridge

Deliver the initial local bridge, Noctalia v4/v5 adapters, desktop client, service unit, tests, and implementation documentation for persistent Pi status and control.

specs/planning-context.yaml

@@ -0,0 +1,27 @@
+feature_name: tauri-pi-desktop-client
+problem_statement: >
+  The current Noctalia panel is not reliably interactive or testable, while
+  users need a feature-equivalent Pi desktop client that works independently
+  of any status bar.
+constraints:
+  - The bridge remains the only Pi process/session/recovery owner.
+  - The Tauri host uses only the local Unix bridge client and never opens TCP.
+  - Noctalia is an optional compact launcher/status compatibility layer.
+  - Folder selection is explicit and canonicalized by the bridge.
+  - Pi owns model, thinking, queue behavior, approvals, and extension policy.
+  - TUI-only custom extension UI must show unsupported and never auto-approve.
+out_of_scope:
+  - Pixel-identical terminal TUI rendering.
+  - Arbitrary custom TUI extension component rendering.
+  - Auto-starting every remembered non-Home folder at login.
+key_decisions:
+  - decision: One default composer
+    rationale: Mirror the confirmed TUI workflow without bridge-owned prompt/steer decisions.
+  - decision: Tauri owns the primary UI
+    rationale: Native host IPC plus frontend/unit/E2E testing is more reliable than a declarative status-bar panel.
+  - decision: Noctalia is a compatibility shim
+    rationale: It displays compact status and opens/focuses the standalone client without owning UI behavior.
+  - decision: Persist remembered folders and last selection
+    rationale: Remove manual ID/folder setup while preserving explicit selection.
+  - decision: Home agent is always running
+    rationale: Provide an immediately available default Pi instance.