feat(agent): add public campus tools

[SebastianBoehler]

Summary

• add typed public capability results with source, freshness, privacy, and effect metadata
• expose get_mensa_options and search_campus_locations through the shared local/cloud tool catalog and executor
• reuse the existing Mensa and Nominatim clients with bounded results, TTL caches, stale-data fallback, validation, redacted errors, and HTTP timeouts
• keep this backend/tool-only and independent of UI work or PR feat(campus): improve Mensa menu card visualization #42

Validation

• flutter analyze
• flutter test (116 tests)
• focused public-tool, executor, and cloud-client tests (24 tests)
• flutter build web --release
• git diff --check

Scope

This is the first implementation slice of #60. It establishes the capability/tool foundation and two public read-only tools; private local tools and user-approved side effects remain follow-up work.

Part of #60.