feat(cli): add on_session_finalize and on_session_reset plugin hooks

Plugins can now subscribe to session boundary events via
ctx.register_hook('on_session_finalize', ...) and
ctx.register_hook('on_session_reset', ...).

on_session_finalize — fires during CLI exit (/quit, Ctrl-C) and
before /new or /reset, giving plugins a chance to flush or clean up.

on_session_reset — fires after a new session is created via
/new or /reset, so plugins can initialize per-session state.

Closes #5592

tests/test_session_boundary_hooks.py

@@ -0,0 +1,66 @@
+import pytest
+from unittest.mock import MagicMock, patch
+from hermes_cli.plugins import VALID_HOOKS, PluginManager
+import os
+import shutil
+import tempfile
+from cli import HermesCLI
+
+
+def test_session_hooks_in_valid_hooks():
+    """Verify on_session_finalize and on_session_reset are registered as valid hooks."""
+    assert "on_session_finalize" in VALID_HOOKS
+    assert "on_session_reset" in VALID_HOOKS
+
+
+@patch("hermes_cli.plugins.invoke_hook")
+def test_session_finalize_on_reset(mock_invoke_hook):
+    """Verify on_session_finalize fires when /new or /reset is used."""
+    cli = HermesCLI()
+    cli.agent = MagicMock()
+    cli.agent.session_id = "test-session-id"
+
+    # Simulate /new command which triggers on_session_finalize for the old session
+    cli.new_session(silent=True)
+
+    # Check if on_session_finalize was called for the old session
+    mock_invoke_hook.assert_any_call(
+        "on_session_finalize", session_id="test-session-id", platform="cli"
+    )
+    # Check if on_session_reset was called for the new session
+    mock_invoke_hook.assert_any_call(
+        "on_session_reset", session_id=cli.session_id, platform="cli"
+    )
+
+
+@patch("hermes_cli.plugins.invoke_hook")
+def test_session_finalize_on_cleanup(mock_invoke_hook):
+    """Verify on_session_finalize fires during CLI exit cleanup."""
+    import cli as cli_mod
+
+    mock_agent = MagicMock()
+    mock_agent.session_id = "cleanup-session-id"
+    cli_mod._active_agent_ref = mock_agent
+    cli_mod._cleanup_done = False
+
+    cli_mod._run_cleanup()
+
+    mock_invoke_hook.assert_any_call(
+        "on_session_finalize", session_id="cleanup-session-id", platform="cli"
+    )
+
+
+@patch("hermes_cli.plugins.invoke_hook")
+def test_hook_errors_are_caught(mock_invoke_hook):
+    """Verify hook exceptions are caught and don't crash the agent."""
+    mgr = PluginManager()
+
+    # Register a hook that raises
+    def bad_callback(**kwargs):
+        raise Exception("Hook failed")
+
+    mgr._hooks["on_session_finalize"] = [bad_callback]
+
+    # This should not raise
+    results = mgr.invoke_hook("on_session_finalize", session_id="test", platform="cli")
+    assert results == []