feat(llm-pi-ai): dormant bare mount — routes live entirely in the settings plane

An empty or omitted providers dict is now the valid dormant posture: the
adapter mounts with zero routes and no catalog entries, registers routes
the moment the llm-pi-ai settings section supplies profiles, and drops
them when it empties. The TUI demo mounts the adapter bare, so adding an
openai/anthropic provider is purely a settings.yaml (or, next PR, web
form) operation with per-request apiKeyEnv credential resolution.

packages/llm/llm-pi-ai/tests/adapter.spec.ts

@@ -395,7 +395,9 @@ describe('provider profile lifecycle', () => {
   })
 
   it('validates empty, unknown, legacy-shaped, and explicitly blank profiles', () => {
-    expect(() => resolveProfiles({})).toThrow(/at least one/)
+    // Empty and omitted dicts are the dormant zero-route posture, not errors.
+    expect(resolveProfiles({}).size).toBe(0)
+    expect(resolveProfiles(undefined).size).toBe(0)
     expect(() => resolveProfiles({ '': {} })).toThrow(/non-empty/)
     expect(() => resolveProfiles({ 'not-real': {} })).toThrow(/unknown/)
     // The pre-release array shape and its per-profile provider field fail