Add agentic opportunity matching MVP

[sajeetharan]

Closes #237

Summary

• add expiring, self-declared opportunity preferences for claimed developers
• display active opportunity intent on public profiles
• let MCP agents filter by opportunity type with explainable matches
• preserve existing verified-agent authentication and developer-approved introductions
• add the agentic opportunity matching PRD and agent documentation

Safety

• private, disabled, expired, or unreachable preferences are omitted from public projections
• no private contact data, autonomous outreach, job scraping, or automatic applications

Validation

pm test (187 passing)

pm run build

• git diff --check
• focused opportunity/profile/MCP tests (17 passing)
• Playwright owner-flow and public-profile checks at 390px with no horizontal overflow