feat(tools): add LRU cache for read-only tool results

[Mattheinrichs]

Summary

Introduces opt-in, in-session LRU caching for read-only tool results. When enabled, repeated identical calls to idempotent tools (file reads, glob, grep, web fetch, MCP resources/prompts) return the cached result rather than re-executing the tool. This reduces redundant work and cuts token usage when the same resources are accessed multiple times in a session.

The feature is off by default and has no effect on existing behaviour unless explicitly enabled.

Changes

File	What changed
nanobot/config/schema.py	Added cacheToolResults (bool, default false) and cacheToolResultsMaxSize (int, default 128) to ToolsConfig
nanobot/agent/tools/registry.py	Added execute_cached() (LRU via OrderedDict) and clear_result_cache(); only caches read_only=True tools; never caches error results
nanobot/agent/runner.py	Route _run_tool() through execute_cached() instead of calling tool.execute() directly
nanobot/agent/loop.py	Pass cache_results/cache_max_size from ToolsConfig into ToolRegistry constructor
tests/tools/test_tool_registry_cache.py	9 async unit tests: disabled-by-default, cache hit, param isolation, key-order normalisation, mutable-tool bypass, error-not-cached, LRU eviction, LRU order refresh, cache clear
README.md	New Tool Result Cache section with config table and JSON example

Design decisions

• Opt-in - zero behaviour change unless cacheToolResults: true is set in config.
• read_only gate - only tools that declare read_only = True are eligible; write/exec tools are never cached.
• Errors not cached - if a tool returns a string starting with 'Error:' the result is discarded so a retry can succeed.
• LRU eviction - OrderedDict.move_to_end / popitem(last=False) gives O(1) LRU with no extra dependency.
• Cache key - (tool_name, json.dumps(params, sort_keys=True)) so parameter order from the LLM does not affect hit rate.
• In-memory, per-session - cache lives with the ToolRegistry instance; no persistence or cross-session leakage.

Testing

uv run pytest tests/tools/test_tool_registry_cache.py -v  # 9 passed
uv run pytest tests/ -q                                    # 1924 passed, 12 skipped

One pre-existing test failure (test_exec_timeout_parameter) uses the sleep shell command which does not exist on Windows; unrelated to this PR.