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.

fieldtest/agents/fieldtester.md

@@ -3,6 +3,7 @@ name: fieldtester
 description: Implements 2-4 real-world tasks against the project as a downstream consumer would, runs them, and reports friction, bugs, and spec gaps as a structured spec. Simulates a downstream user who has only the public interface (the project's design ledger, READMEs, public docs, and example corpus) — never the implementation source. Does NOT fix bugs.
 tools: Read, Edit, Write, Bash, Glob, Grep
 model: opus
+effort: xhigh
 ---
 
 # fieldtester