P0: add explore facade

[lzehrung]

Summary

• add library, CLI, and MCP explore facade over existing search, packet, path, and reverse-dependency primitives
• return bounded schemaVersion 1 responses with anchors, packets, paths, blast radius, candidate tests, follow-ups, limits, and omissions
• document explore in README, CLI, MCP, library API, agent workflows, and bundled skill

Verification

• npm run build
• npx vitest run tests/agent-explore.test.ts
• npm run check
• odw review-and-correct: clean, 0 confirmed findings

Stacked on #142 / p0/mcp-freshness-on-query.