fix(opencode): reject Rust src/tests/ paths as a wrong task spec

A workflow run on a Bevy/Rust project produced the test-file path
`src/tests/test_<feature>.rs`, which @test correctly flagged as
contradictory: it isn't a valid Rust test location (would require
declaring `mod tests;` in production source, which @test cannot do)
yet the file-gate glob `**/tests/**/*.rs` accidentally matched it.

Phase 5 now gives language-aware Test File guidance: Python uses
colocated or top-level `tests/`, Rust uses crate-level `tests/<feature>.rs`,
and Rust unit-only tasks are routed to NOT_TESTABLE for @make to
handle inline. Phase 6's file gate gains an explicit anti-pattern
clause discarding any new file under `src/` even when the glob matches.

@test's own File Constraint mirrors the anti-pattern so the agent
rejects the bad path with BLOCKED before the orchestrator's gate
even runs — defense in depth on both sides of the dispatch boundary.

config/opencode/agents/test.md

@@ -119,6 +119,9 @@ Rust (integration tests only — see "Rust unit tests" below):
 - `**/test_data/**`
 - `**/test_fixtures/**`
 
+**Anti-patterns — refuse the path even if the glob above matches:**
+- Anything under `src/` (e.g. `src/tests/foo.rs`, `src/**/tests/...`). `src/tests/` is a regular module under `src/`; it would require declaring `mod tests;` in production code (`lib.rs` / `main.rs`) and creating `mod.rs`, which you cannot do. If the caller asks for such a path, treat it as a wrong task spec: return `BLOCKED` with a note that the path is not a valid Rust test location, suggesting `tests/<feature>.rs` (or `NOT_TESTABLE: Rust unit-only` if the test really needs to be in-source).
+
 **You may NOT modify production/source code under any circumstances.**
 
 ### Rust unit tests