feat(chat): add agent orchestration system with plan_and_execute

Introduces TaskPlanner and AgentSpec so Pyra can decompose multi-step
tasks into sequential steps, each executed with a focused sub-agent
context rather than the full conversation history.

- plugins/base.py: AgentSpec dataclass + agent_spec() on Protocol/BasePlugin
- plugins/registry.py: register_builtin, get_agent, list_agents
- chat/planner.py: TaskPlanner with plan approval, per-step tool-use loop,
  verification call, and agent-aware routing
- chat/session.py: wires plan_and_execute as a built-in tool after load_all
- chat/history.py: planning hint in system prompt + dynamic agents listing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pyra/plugins/registry.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from pathlib import Path
 from typing import Callable, Coroutine
 
-from pyra.plugins.base import PyraPlugin, Tool
+from pyra.plugins.base import AgentSpec, PyraPlugin, Tool
 from pyra.plugins.loader import _log_error, load_plugins
 from pyra.vault.reader import get_key
 
@@ -77,3 +77,25 @@ class PluginRegistry:
 
     def find_tool(self, name: str) -> Tool | None:
         return self._tools.get(name)
+
+    def register_builtin(self, tool: Tool) -> None:
+        """Register a built-in tool independent of plugins. Call after load_all."""
+        self._tools[tool.name] = tool
+
+    def get_agent(self, name: str) -> tuple[AgentSpec, list[Tool]] | None:
+        """Return (AgentSpec, tools) for a named plugin agent, or None."""
+        plugin = self._plugins.get(name)
+        if plugin is None:
+            return None
+        spec = plugin.agent_spec()
+        if spec is None:
+            return None
+        return (spec, plugin.tools())
+
+    def list_agents(self) -> list[tuple[str, AgentSpec]]:
+        """Return (plugin_name, AgentSpec) for all plugins that have agents."""
+        return [
+            (name, plugin.agent_spec())
+            for name, plugin in self._plugins.items()
+            if plugin.agent_spec() is not None
+        ]