Add MCP dynamic tool refresh with push notifications and periodic pol… (#31)

* Add MCP dynamic tool refresh with push notifications and periodic polling

MCP tool lists now stay up-to-date without restart via three mechanisms:
push notifications (ToolListChangedNotification) for servers that support
it, staggered periodic polling for servers that don't, and manual
/mcp refresh [server] command. MCPClientManager tracks tools per-server
with copy-on-write rebuild, notifies ChatSession listeners which rebuild
tool lists and ToolSearchManager (preserving expanded tools).

* Address Copilot review feedback on MCP refresh PR

- Fix /mcp refresh typo matching (startswith → exact token check)
- Validate --mcp-refresh-interval >= 0 at parse time via shared
  nonneg_float in config.py (deduplicated from cli.py + server.py)
- Clamp negative refresh_interval to 0 in MCPClientManager constructor
- Fix periodic refresh first poll timing (was initial_delay + interval,
  now initial_delay then immediate first poll)
- Clarify _on_mcp_tools_changed docstring re: O(n) BM25 build cost

docs/diagrams/png/03-core-engine-classes.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:8f47555af0ff16c8a236a3edcb0d75ce1b2850d3ecb3cc4627526d5b9890327a
-size 522131
+oid sha256:4512be48a51f7cd1136ea8e3344489a8d225c611cb1b961643b869351de76812
+size 549668