refactor(agent-runner): decouple provider interface from Claude specifics

Reshape AgentProvider so provider-specific assumptions stop leaking into
the generic layer. No change to what reaches sdkQuery() — same values,
different plumbing.

- QueryInput: opaque `continuation` replaces `sessionId` + `resumeAt`;
  `systemContext.instructions` replaces ambiguous `systemPrompt`;
  `mcpServers`, `env`, `additionalDirectories` move to `ProviderOptions`
  at construction time.
- AgentProvider gains `isSessionInvalid(err)` and
  `supportsNativeSlashCommands` so the poll-loop stops regex-matching
  Claude error strings and gates passthrough slash commands per provider.
- ClaudeProvider owns `CLAUDE_CODE_AUTO_COMPACT_WINDOW` and the
  stale-session regex internally.
- ProviderEvent.activity kept and documented as the liveness signal
  (fires on every SDK message so the idle timer stays honest during
  long tool runs); init carries `continuation` instead of `sessionId`.
- poll-loop drops mcpServers/env/systemPrompt from its config; admin
  user id now passed explicitly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/poll-loop.test.ts

@@ -104,12 +104,10 @@ describe('routing', () => {
 
 describe('mock provider', () => {
   it('should produce init + result events', async () => {
-    const provider = new MockProvider((prompt) => `Echo: ${prompt}`);
+    const provider = new MockProvider({}, (prompt) => `Echo: ${prompt}`);
     const query = provider.query({
       prompt: 'Hello',
       cwd: '/tmp',
-      mcpServers: {},
-      env: {},
     });
 
     const events: Array<{ type: string }> = [];
@@ -127,12 +125,10 @@ describe('mock provider', () => {
   });
 
   it('should handle push() during active query', async () => {
-    const provider = new MockProvider((prompt) => `Re: ${prompt}`);
+    const provider = new MockProvider({}, (prompt) => `Re: ${prompt}`);
     const query = provider.query({
       prompt: 'First',
       cwd: '/tmp',
-      mcpServers: {},
-      env: {},
     });
 
     const events: Array<{ type: string; text?: string }> = [];
@@ -164,12 +160,10 @@ describe('end-to-end with mock provider', () => {
     const prompt = formatMessages(messages);
 
     // Create mock provider and run query
-    const provider = new MockProvider(() => 'The answer is 4');
+    const provider = new MockProvider({}, () => 'The answer is 4');
     const query = provider.query({
       prompt,
       cwd: '/tmp',
-      mcpServers: {},
-      env: {},
     });
 
     // Process events — simulate what poll-loop does