feat(agents): give Pi tool registrations their full description

[vitaliyslion]

Summary

Brings the Pi extension's tool descriptions to parity with what real MCP clients already receive: gortex tools search gains a --format json mode, and ToolDescriptor (Go + CLI mirror) now carries the full untruncated Description alongside the existing first-sentence Summary. Pi's registerOneTool uses it directly instead of synthesizing a shortened description — removing a Pi-specific truncation that no other MCP agent was subject to.

Changes

• Add Description field to mcp.ToolDescriptor and cmd/gortex's toolDescriptorCLI mirror, populated via strings.TrimSpace(e.desc).
• Add --format json to gortex tools search, emitting {name, description} per match instead of the truncated single-line text listing.
• Update the Pi extension (internal/agents/pi/extension/index.ts) to call tools search --format json, parse it with a new parseToolSearchJSON/firstSentence, and register each tool with its full description (falling back to summary, then name) instead of a hardcoded "(Gortex graph tool — prefer over raw file reads.)" suffix.

Testing

• All tests pass (go test ./internal/mcp/... ./cmd/gortex/...)
• New tests added for new functionality
• Benchmarks run if performance-relevant (not performance-relevant)

Checklist

• Code follows existing patterns in the codebase
• No unnecessary abstractions added
• Language extractor includes Meta["methods"] for interfaces (if applicable)
• Methods have EdgeMemberOf edges to their containing type (if applicable)