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>

pyproject.toml

@@ -105,6 +105,9 @@ classifiers = [
     "Topic :: Communications :: Chat",
 ]
 
+[tool.uv.sources]
+langbot-plugin = { path = "../langbot-plugin-sdk", editable = true }
+
 [project.urls]
 Homepage = "https://langbot.app"
 Documentation = "https://docs.langbot.app"
@@ -223,4 +226,3 @@ skip-magic-trailing-comma = false
 
 # Like Black, automatically detect the appropriate line ending.
 line-ending = "auto"
-