v1/mode: task_class → mode/model router (decision-only, phase 1)

HANDOVER §queued (2026-04-25): "Mode router — port LLM Team multi-model
patterns. Pick the right TOOL/MODE for each task class via the matrix,
not cascade through models."

Two-stage architecture:
  1. Decision (POST /v1/mode) — pure recommendation, no execution.
     Returns {mode, model, decision: {source, fallbacks, matrix_corpus,
     notes}} so callers see WHY this mode was picked.
  2. Execution (future POST /v1/mode/execute) — proxy to LLM Team
     /api/run for modes not yet ported to native Rust runners. Not
     wired in this phase.

Splitting decision from execution lets us A/B-test the routing logic
without committing to running every recommendation. The decision
function is pure enough for exhaustive unit tests (3 added).

config/modes.toml — initial map for 5 task_classes (scrum_review,
contract_analysis, staffing_inference, fact_extract, doc_drift_check)
+ a default. matrix_corpus per task is reserved for the future
matrix-informed routing pass.

VALID_MODES list (24 modes) is kept in sync manually with LLM Team's
/api/run handler at /root/llm_team_ui.py:10581. Adding a mode here
without adding it upstream returns 400 from a future proxy.

GET /v1/mode/list — operator introspection so a UI can render the
registry table without re-parsing TOML.

Live-tested: 5 task classes match, unknown classes fall through to
default, force_mode override works + validates, bogus modes return
400 with the valid_modes list.

Updates reference_llm_team_modes.md memory — earlier note claiming
"only extract is registered" was wrong (all 25 are registered).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/v1/mod.rs

@@ -17,6 +17,7 @@ pub mod openrouter;
 pub mod gemini;
 pub mod claude;
 pub mod langfuse_trace;
+pub mod mode;
 pub mod respond;
 pub mod truth;
 
@@ -83,6 +84,8 @@ pub fn router(state: V1State) -> Router {
         .route("/usage", get(usage))
         .route("/sessions", get(sessions))
         .route("/context", get(truth::context))
+        .route("/mode", post(mode::route))
+        .route("/mode/list", get(mode::list))
         .with_state(state)
 }