Add dynamic tool search with native defer_loading for Anthropic/OpenAI (#30)

* Add dynamic tool search with native defer_loading for Anthropic/OpenAI

When MCP tools push the total tool count past a configurable threshold
(default 20), tool definitions are deferred to reduce token overhead and
improve tool selection accuracy. Three-tier approach mirrors the existing
web search pattern:

- Anthropic (Claude 4.x): native defer_loading + server-side BM25 search
- OpenAI (GPT-5.4+): native defer_loading + hosted search
- vLLM/llama/NIM: client-side BM25 fallback via synthetic tool_search tool

New module turnstone/core/tool_search.py with BM25Index (pure-Python,
zero deps) and ToolSearchManager (session-scoped visibility, expansion,
server hint generation). Discovered tools persist for the session lifetime
so the model only searches once per capability needed.

Config: [tools] search/search_threshold/search_max_results
CLI: --tool-search {auto,on,off}, --tool-search-threshold, --tool-search-max-results
Agents (plan/task) exempt — their scoped tool sets are always small.

43 new tests (1253 total). All diagrams regenerated with PlantUML 1.2025.2.

* Fix Copilot review feedback on tool search

- Fix _MCP_PREFIX_RE to handle underscores in server names (non-greedy match)
- Use ordered dict for _expanded to preserve tool discovery order
- Avoid constructing ToolSearchManager when below threshold in auto mode
- Return empty string from _mcp_server_summary when no servers (not "none")
- Fix CLI help text to reference threshold generically, not hardcoded "20"
- Fix agent exemption docs to accurately describe scoped tool sets
- Fix README to not hardcode "30+" threshold number

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

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