register trace as sixth MCP tool (PR-TRACE-2)

[HumanBean17]

Summary

• Register trace as the sixth MCP tool in create_mcp_server() with complete description matching propose § "Agent tool selection" guidance
• Update _INSTRUCTIONS to list six tools (search, find, describe, neighbors, resolve, trace)
• Wire asyncio.to_thread to mcp_trace.trace_v2 with all parameters from the propose § "Signature"
• Add mcp_trace to py-modules in pyproject.toml (required for CLI subprocess import resolution)
• Update test_mcp_tools.py to expect 6 tools including trace

Scope

Implements PR-TRACE-2 from plans/active/PLAN-TRACE-TOOL.md.

Manual evidence

.venv/bin/python -c "
from server import create_mcp_server
srv = create_mcp_server()
tools = [t.name for t in srv._tool_manager._tools.values()]
print('tools:', tools)
assert 'trace' in tools, 'trace not registered'
print('ok: trace registered as 6th tool')
"

Output: tools: ['search', 'find', 'describe', 'neighbors', 'resolve', 'trace']

Intentional scope expansion

• pyproject.toml — added mcp_trace to py-modules list. Without this, CLI subprocess tests fail with No module named 'mcp_trace' because server.py imports mcp_trace and the package didn't include it.
• tests/test_mcp_tools.py — updated test_registered_tool_surface_is_v2_navigation_only to expect 6 tools. Without this, the existing test fails because it asserts exactly 5 tools.

Out of scope (not touched)

• mcp_trace.py — no changes (empty diff confirmed)
• mcp_v2.py, kuzu_queries.py — no changes
• build_ast_graph.py, java_ontology.py — no changes
• mcp_hints.py — no changes (PR-TRACE-3)
• skills/explore-codebase/SKILL.md — no changes (PR-TRACE-3)

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