feat(agents): pin explicit reasoning effort on every agent and workflow call

Effort joins model as a mandatory pin: an omitted field inherits the
session effort, coupling every dispatch's thinking budget to whatever
the user happens to be chatting at (often xhigh) — the same
session-state coupling the model pin removes. The assignment follows
the model split:

- xhigh on every opus agent (judgement roles are the pipeline's
  quality floor and must not degrade with the session);
- high on every sonnet agent (tightly-scoped plan execution gains
  little from xhigh but pays its latency per dispatch, and these are
  the per-task in-loop roles — wall-clock is the efficiency metric;
  not lower than high, since re-loops cost more than saved thinking);
- medium inline in the workflow scripts for schema-bound
  extraction/verification stages that author no code (preflight,
  plan-extract, mini-verify, tree-check, finalize, build/suite
  verify).

Workflow agent() calls pass effort explicitly on every call — whether
frontmatter effort propagates through an agentType dispatch is
undocumented, so the scripts do not rely on it. Policy documented in
docs/agent-template.md § effort, mirroring § model.

implement/agents/implementer.md

@@ -3,6 +3,7 @@ name: implementer
 description: Carries out a tightly scoped implementation task. Reads the task extract handed by the controller, implements, builds, tests, reports a structured status with the diff. NOT for architecture decisions, NOT for self-curated scope; this agent executes a plan that has already been made.
 tools: Read, Edit, Write, Bash, Glob, Grep
 model: sonnet
+effort: high
 ---
 
 # implementer