MCP contract: add structured outputs and explainable discovery results

[sajeetharan]

Problem

AI agents require clear, structured outputs and explainable matches when performing developer discovery searches on DevGlobe to build trust in recommendations.

Acceptance Criteria

• Add outputSchema and structuredContent to all public discovery tools.
• Search result items must include: profileUrl, whyMatched, public evidence, freshness, agent availability, and methodology disclaimer.
• Maintain backward-compatible text content outputs.
• Comprehensive test coverage.

Dependencies

• None.

[sajeetharan]

The implementation for this issue has been completed in draft PR #204.
Delivered Scope: Rich semantic output formats via structuredContent / outputSchema, with clear reasoning, match scores, and evidence for developer matches ('whyMatched').
Validation: Passes tests and local MCP JSON-RPC profile calls successfully return high-fidelity structured data.

[sajeetharan]

Shipped in PR #204 and was verified on production.