feat(agent-runner): integrate AgentRunner Protocol v1 with plugin system

Phase 0 integration complete - verified minimal loop with local-agent stub runner.

Changes:
- Add AgentRunOrchestrator for plugin-based agent execution
- Add AgentResultNormalizer for Protocol v1 result conversion
- Add AgentRunnerDescriptor for runner ID parsing (plugin:author/name/runner)
- Update chat handler to use new orchestrator instead of direct runner lookup
- Add plugin handler methods for list_agent_runners and run_agent
- Add connector methods for AgentRunner protocol forwarding
- Update pipeline API to include runner options in metadata
- Add integration docs and implementation plan

Integration verified:
- Runner: plugin:langbot/local-agent/default
- Input: "你好"
- Output: [stub] Echo: 你好
- Date: 2026-05-10 10:09

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/langbot/pkg/agent/runner/id.py

@@ -0,0 +1,92 @@
+"""Agent runner ID parsing and formatting."""
+from __future__ import annotations
+
+import dataclasses
+
+
+@dataclasses.dataclass(frozen=True)
+class RunnerIdParts:
+    """Parsed runner ID components."""
+    source: str  # 'plugin' (future: 'builtin')
+    plugin_author: str
+    plugin_name: str
+    runner_name: str
+
+    def to_plugin_id(self) -> str:
+        """Return plugin identifier as author/name."""
+        return f'{self.plugin_author}/{self.plugin_name}'
+
+
+def parse_runner_id(runner_id: str) -> RunnerIdParts:
+    """Parse runner ID string into components.
+
+    Args:
+        runner_id: Runner ID in format 'plugin:author/plugin_name/runner_name'
+
+    Returns:
+        RunnerIdParts with parsed components
+
+    Raises:
+        ValueError: If runner_id format is invalid
+    """
+    if runner_id.startswith('plugin:'):
+        parts = runner_id[7:].split('/')
+        if len(parts) != 3:
+            raise ValueError(
+                f'Invalid plugin runner ID format: {runner_id}. '
+                f'Expected: plugin:author/plugin_name/runner_name'
+            )
+        plugin_author, plugin_name, runner_name = parts
+        if not plugin_author or not plugin_name or not runner_name:
+            raise ValueError(
+                f'Invalid plugin runner ID: {runner_id}. '
+                f'author, plugin_name, and runner_name must be non-empty'
+            )
+        return RunnerIdParts(
+            source='plugin',
+            plugin_author=plugin_author,
+            plugin_name=plugin_name,
+            runner_name=runner_name,
+        )
+    else:
+        # For backward compatibility with old built-in runner names
+        # This should eventually be removed after migration
+        raise ValueError(
+            f'Invalid runner ID format: {runner_id}. '
+            f'Expected: plugin:author/plugin_name/runner_name'
+        )
+
+
+def format_runner_id(
+    source: str,
+    plugin_author: str,
+    plugin_name: str,
+    runner_name: str,
+) -> str:
+    """Format runner ID from components.
+
+    Args:
+        source: Runner source ('plugin')
+        plugin_author: Plugin author
+        plugin_name: Plugin name
+        runner_name: Runner component name
+
+    Returns:
+        Runner ID string
+    """
+    if source == 'plugin':
+        return f'plugin:{plugin_author}/{plugin_name}/{runner_name}'
+    else:
+        raise ValueError(f'Invalid runner source: {source}')
+
+
+def is_plugin_runner_id(runner_id: str) -> bool:
+    """Check if runner ID is a plugin runner.
+
+    Args:
+        runner_id: Runner ID string
+
+    Returns:
+        True if runner ID starts with 'plugin:'
+    """
+    return runner_id.startswith('plugin:')