Remove built-in agent dynamic context

[gold-silver-copper]

What changed

• remove AgentBuilder::dynamic_context, ExtractorBuilder::dynamic_context, DynamicContextStore, and all runner/request plumbing
• migrate passive-RAG examples to small application-owned AgentHook::on_completion_call implementations
• preserve retrieved-tool query selection and the vector-index Tool implementation
• document passive hook-based RAG versus active tool-based RAG
• add regression coverage for blocking/streaming hook context, retrieval failure, extractor hooks, query selection, vector tools, and source-tree API removal

Why

Hardcoded passive retrieval embedded application policy in agent request construction. Hooks already provide the correct per-turn policy boundary and behave consistently across blocking, streaming, and extractor execution.

Impact

This is a breaking API removal. Callers should replace .dynamic_context(samples, index) with a local completion-call hook that queries the index and patches RequestPatch::extra_context. Retrieval failures can stop the run before provider I/O. Model-directed retrieval remains available by exposing a vector index or custom retriever as a tool.

Root cause

The original dynamic-context feature coupled vector retrieval, query selection, document formatting, and error policy to the core completion builder. Removing that special path leaves one managed execution lifecycle and makes retrieval policy explicit.

Verification

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo doc --workspace --no-deps
• cargo check -p rig-core --target wasm32-unknown-unknown --features wasm
• targeted migrated-example checks and independent full-diff review

External-service-backed examples were compiled but not executed.