feat(mcp): add compact descriptor mode

[bertheto]

Summary

• Adds opt-in MCP_DESCRIPTOR_MODE=compact for shorter MCP meta-tool descriptions.
• Keeps verbose as the default so existing users retain the current inline guidance.
• Preserves the three meta-tool names, schemas, registry behavior, and execution payloads.

Why

Some MCP clients load tool descriptions into the model context at startup. Forgetful already uses the right meta-tool pattern, but the discover_forgetful_tools and execute_forgetful_tool descriptions embed long examples and catalogs. Compact mode lets context-sensitive clients keep those descriptors small while still fetching full schemas/examples on demand via how_to_use_forgetful_tool.

Validation

• uv run pytest tests/integration/test_meta_tools_docstrings.py -> 25 passed.
• uv run python -m compileall app/routes/mcp/meta_tools.py app/config/settings.py -> passed.
• Local compact descriptor check via main.mcp.list_tools() -> 3 tools, 1,442 total description chars (discover 506, how_to_use 321, execute 615).
• uv run pytest tests/e2e_sqlite/test_meta_tools_sqlite.py was attempted but blocked during setup because FastEmbed attempted a HuggingFace model download and the remote connection was closed.

Compatibility

• Default behavior remains MCP_DESCRIPTOR_MODE=verbose.
• Unknown descriptor mode values fall back to verbose.
• No inner tool schemas, adapters, registry behavior, or response payloads are changed.