perf(agents): streamline Terra execution

Keep Sol responsible for scope, steering, and final acceptance while assigning one continuous discovery, implementation, testing, and iteration unit to Terra. Route additional reviewers and runtime verification by risk and avoid redundant verification passes.

AGENTS.md

@@ -25,13 +25,16 @@ MCP service in one npm/Turborepo workspace.
    and prove the relevant compatibility contracts still hold. Preserve deployed
    interfaces and supported client behavior; repository checks alone do not
    prove a live change is safe.
-10. Delegate every substantive repository task to at least one project agent so
-   detailed exploration or execution stays out of the primary context.
-   "Substantive" includes repository exploration, multi-file reasoning,
-   implementation, test execution, or contract/risk analysis. Use `repo_scout`
-   for read-only investigation and `implementation` for bounded changes; use
-   the additional roles according to `docs/agents/routing.md`. The primary
-   agent retains scope, acceptance, review, and the final answer. Skip
+10. The Sol primary agent owns scope, risk classification, acceptance criteria,
+   progress steering, final diff and evidence review, and the final answer.
+   Delegate every substantive repository task to a project agent. Each
+   substantive implementation must use one Terra `implementation` agent as a
+   coherent execution unit, normally combining scoped discovery, implementation,
+   focused tests, and iteration. "Substantive" includes repository exploration,
+   multi-file reasoning, implementation, test execution, or contract/risk
+   analysis. Add a `repo_scout`, reviewer, or verifier only for separable work or
+   the triggers in `docs/agents/routing.md`; do not duplicate the implementation
+   agent's detailed exploration or checks without a concrete reason. Skip
    delegation only for trivial conversation, a single obvious lookup, or a
    mechanical one-line edit whose coordination cost exceeds the work.
 11. Use high-effort reviewers only for the risk triggers in