refactor(examples): unify full-flow configs into B-shaped gateway

[leseb]

Summary

Merge the standalone full-flow.yaml gateway into full-flow-agentic.yaml as a single "B-shaped" config: POST /v1/responses runs through the terminal iterative_request_router for server-side file_search, while the Responses WebSocket (GET /v1/responses) and the non-Responses paths (/v1/prompts, /v1/embeddings, /v1/files, /v1/vector_stores) branch around the IRR via a conditions-gated headers host, and a format-first guard rejects Chat Completions / Anthropic Messages bodies on /v1/responses with 404. This is the smallest complete change because it deletes the now-redundant full-flow.yaml, migrates every test and doc reference to the unified config in one pass, and regenerates the examples README. The config is a draft gated on #313 (file_search SSE streaming lifecycle); an #[ignore]d streaming placeholder test marks the post-#313 expectation.

Related issue

Refs #313

Validation

• Unit tests — no production code changed (example-config + test/doc migration only)
• Integration or functional tests — cargo test -p praxis-tests-integration --test suite green for full_flow (12), full_flow_agentic (6 + 1 ignored, file_search: SSE event emission for progress streaming #313), session_replay (9), codex_websocket (2); cargo xtask lint-example-tests → 41 configs covered
• make lint (exit 0) and make build (exit 0)

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test.
• User-facing behavior and generated documentation are updated.
• Performance-sensitive changes include appropriate benchmark or load-test evidence. (N/A — no production/perf code changed)
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

Deletes the full-flow.yaml example config; external references must switch to full-flow-agentic.yaml. This is a draft — do not merge until #313 lands and validates the file_search SSE streaming lifecycle.