file_search: core filter implementation

[leseb]

Parent

Part of #29

Scope

Implement the core file_search filter that executes file_search tool calls against vector store APIs.

Included

• Filter module with HttpFilter implementation (response-phase)
• Config struct: vector_store_url, api_key, auth_type, templates, timeout_ms
• CalloutClient from praxis-core for HTTP calls with circuit breaking
• Parallel fan-out across multiple vector stores
• Simple {variable} template interpolation for search/annotation/context formatting
• Citation extraction: <|file-xxx|> markers → annotation structs
• citation_files tracking (file_id → filename)
• Unit tests + integration tests + example config

Not included

• SSE event emission (file_search_call.in_progress, .searching, .completed) — see sibling issue