feat: P1+P2 backend completion — 37 endpoints across 9 modules

[ULookup]

Summary

• P1 (18 endpoints): agent-driven generation, agent search/filter, character appearances, workspace file operations
• P2 (19 endpoints): pipeline control, diary/memory/relations/voice, world export/import, knowledge graph, timeline
• Store-layer upgrades: generalized agent search, kind-filtered agent listing, timeline store methods, export/import service methods
• Integration tests for agent search, kind filter, and character appearances
• Updated SearchAgentTool spec with full-text query and race filter
• API reference docs for all new endpoints

Fix: previously missing P1 files

The initial P2 squash commit (3cb21a5) omitted 11 files from the P1 branch, causing:

• Compilation breakage: handlers calling non-existent search_agents(SearchCriteria) and list_agents(wid, optional<kind>)
• Missing endpoints: 3 workspace file CRUD endpoints (POST/DELETE/POST rename)
• Missing tests: agent endpoint integration tests
• Outdated tool spec: SearchAgentTool still using old traits-only schema

These have been restored via cherry-picks from feat/p1-backend-completion.

Test plan

• Build: cmake --build . passes with zero errors
• Unit tests: ctest passes all existing tests
• New integration tests: agent search, kind filter, character appearances
• Manual smoke: POST/GET /api/worldbuilding/:wid/agents/search with q/traits/race params
• Manual smoke: POST/DELETE/POST rename /api/workspace/files