feat(host): user-invocable skill listing and skill.invoke injection RPC

skill.list now serves every user-invocable skill and carries modelInvocable
so menus can mark user-only entries; the old model-and-user intersection
hid disable-model-invocation skills from their only legitimate entry point
(issue #1470). skill.invoke enforces user-invocation policy at the host
boundary, renders the canonical <skill_content> body, and injects it as a
user-role message carrying the skill-invocation source before starting a
turn. The connection fixture mirrors both faces for client tests.

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -86,7 +86,7 @@ function scriptedApi(overrides: {
       execute: r => ok(r, { matched: false }),
       ...overrides.commands,
     },
-    skills: { list: r => ok(r, { skills: [] }), ...overrides.skills },
+    skills: { list: r => ok(r, { skills: [] }), invoke: r => ok(r, { accepted: true as const }), ...overrides.skills },
     goals: {
       create: err,
       edit: err,