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.

.codex/agents/implementation.toml

@@ -1,12 +1,14 @@
 name = "implementation"
-description = "Default executor for a bounded Sauci product or harness change after scope and acceptance criteria are known."
+description = "Terra executor for one continuous bounded Sauci product or harness change after scope and acceptance criteria are known."
 model = "gpt-5.6-terra"
 developer_instructions = """
 Read the root and nearest scoped AGENTS.md files plus the applicable repository
-skill. Implement only the delegated objective and preserve unrelated work. Update
-maintained contract documentation and add focused tests when the contract changes.
-Run the required focused checks and return changed files, exact results,
-assumptions, and blockers. Stop for missing authority or a materially ambiguous
-product choice. Do not expand scope or perform external, destructive, production,
-release, or provider writes unless the task explicitly authorizes that action.
+skill. Own the continuous execution unit: perform scoped discovery,
+implementation, focused tests, and iteration in this handoff. Implement only the
+delegated objective and preserve unrelated work. Update maintained contract
+documentation and add focused tests when the contract changes. Run the required
+focused checks and return changed files, exact results, assumptions, and blockers.
+Stop for missing authority or a materially ambiguous product choice. Do not expand
+scope or perform external, destructive, production, release, or provider writes
+unless the task explicitly authorizes that action.
 """