refactor(agent): simplify tool disclosure, index tool schemas

[shallyan]

Summary

Progressive tool disclosure previously exposed three meta-tools:
tool_search, tool_describe, and tool_call. tool_describe was a separate
round-trip to fetch a hidden tool's parameter schema after tool_search. This
PR collapses the flow to two meta-tools:

• Drop tool_describe. tool_search now returns each hit's parameter
  schema alongside its name and description, so the model can go straight to
  tool_call. When arguments don't fit the schema the registry already returns
  a correctable validation error, so the extra lookup step added no correctness.
• Index tool parameter schemas. Parameter names, their descriptions, and
  enum values are folded into the BM25 index body (name x3 + description +
  schema keywords). Discriminating keywords often live in the schema rather than
  the one-line description (a repo owner, a channel id, an image size), so
  indexing them lifts retrieval recall without changing the schema the model
  sees. The index signature now keys on (name, description, parameters), so a
  parameter change under hot-reload correctly triggers a rebuild.

Offline validation on the ToolRet benchmark (7,961 queries, 44k-tool corpus,
Raven's own BM25 kernel) shows folding the schema into the index consistently
lifts nDCG@10 by ~4-5 points over indexing name + description alone.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

uv run pytest tests/test_tool_search.py tests/test_agent_loop_tool_search.py -q
# 35 passed

uv run ruff check raven/agent/tools/tool_index.py raven/agent/tools/tool_search.py \
    raven/config/schema.py tests/test_tool_search.py tests/test_agent_loop_tool_search.py
# All checks passed!

uv run ruff format --check raven/agent/tools/tool_index.py raven/agent/tools/tool_search.py \
    tests/test_tool_search.py
# already formatted

New unit coverage: schema-keyword extraction (names, descriptions, enums,
nesting, depth cap, non-dict input), rebuild-on-parameters-change, search hits
carrying parameters, meta-tool set no longer including tool_describe.

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Progressive tool disclosure is opt-in (tools.tool_search.enabled defaults to
false), so the default agent path is unaffected. tool_describe is an internal
meta-tool with no public API; removing it changes only the enabled-feature flow.
tool_search hits are larger now (they carry parameter schemas), but only when
the model actively searches - the per-turn tool list and prompt-cache prefix are
untouched. Rollback: revert the branch.

Related Issues

N/A