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.

docs/agents/routing.md

@@ -1,12 +1,17 @@
 # Agent routing and effort
 
-The primary agent owns scope, acceptance criteria, task decomposition, review,
-and the final answer. Every substantive repository task must delegate at least
-one bounded unit to the best-fit project agent. Use `repo_scout` for read-only
-discovery or analysis and `implementation` for product or harness changes. Add
-other agents only for separable work or the risk triggers below. Skip delegation
-only for trivial conversation, a single obvious lookup, or a mechanical one-line
-edit whose coordination cost exceeds the work.
+Sol is the primary agent and owns scope, risk classification, acceptance
+criteria, task decomposition, progress steering, final diff and evidence review,
+and the final answer. Every substantive repository task must delegate to a
+project agent. Each substantive implementation must delegate one coherent
+execution unit to the Terra `implementation` agent. That handoff normally
+combines scoped discovery, implementation, focused tests, and iteration in one
+continuous task. Add a `repo_scout`, reviewer, test writer, or verifier only for
+separable work or the triggers below. The primary reviews the returned diff and
+evidence, but does not repeat 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.
 
 ## Implementation and integration
 
@@ -44,10 +49,10 @@ Project Codex defaults live in `.codex/config.toml`; task profiles live in
 
 | Role | Codex model | Default effort | Trigger | Do not use for |
 |---|---|---:|---|---|
-| `repo_scout` | Luna | low | locate ownership, callers, contracts | editing or design decisions |
-| `implementation` | Terra | medium, overridable | bounded product or harness change | broad architecture review |
+| `repo_scout` | Luna | low | separable read-only ownership, caller, or contract lookup | editing or design decisions |
+| `implementation` | Terra | medium, overridable | one continuous scoped discovery, implementation, test, and iteration unit | broad architecture review |
 | `test_writer` | Terra | medium | focused regression or missing contract test | rubber-stamping green output |
-| `verifier` | Terra | medium | independently drive implemented behavior | editing implementation |
+| `verifier` | Terra | medium | user-facing, cross-boundary, or high-risk runtime behavior | editing implementation or low-risk package-local changes |
 | `database_reviewer` | Terra | high | migrations, RLS, function/query boundaries | ordinary UI changes |
 | `security_reviewer` | Sol | high | auth, RLS, crypto, secrets, payments, destructive operations | every routine change |
 | `architecture_reviewer` | Sol | high | cross-cutting or hard-to-reverse decisions | local refactors |
@@ -71,13 +76,26 @@ the implementation agent into a premium generalist. Add the matching read-only
 Sol reviewer and keep implementation on Terra unless the user explicitly asks for
 a different model.
 
+## Verification routing
+
+Use a fresh Terra `verifier` after implementation when the change has
+user-facing runtime behavior, crosses an application, API, data, authentication,
+or provider boundary, or is otherwise high risk. The verifier independently
+drives the acceptance criteria and returns evidence only. It is not required for
+documentation, mechanical configuration, isolated tests, or low-risk
+package-local changes with focused deterministic checks. A verifier may still be
+added when independent observation would materially increase confidence.
+
 ## Handoff contract
 
 Every delegated task includes the objective, owned files or surface, acceptance
 criteria, applicable `AGENTS.md` and skill, allowed side effects, required checks,
 and the evidence to return. The agent must report changed files, commands and
-results, assumptions, and blockers. The primary agent independently reviews the
-diff and decides whether acceptance has been met.
+results, assumptions, and blockers. The primary independently reviews the final
+diff and evidence and decides whether acceptance has been met. It should steer
+the active implementation agent with targeted feedback rather than restart the
+same discovery or verification in another agent unless an independent judgment,
+separable workstream, or the verification-routing trigger requires it.
 
 Parallel agents are for independent read-heavy work or non-overlapping ownership.
 Avoid concurrent edits to the same files, shared database state, fixed local