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.

tdd/agents/tdd-author.md

@@ -3,6 +3,7 @@ name: tdd-author
 description: Turns a test-specifiable feature description or issue into a single minimal, autonomous RED executable-spec test ("how it should work"), then hands the GREEN side to implement mini-mode. On a design fork (behaviour not test-specifiable) it bounces to brainstorm; on a GREEN one iteration can't reach, it decomposes the headline into BLOCKER sub-tests. Does NOT implement the feature.
 tools: Read, Edit, Write, Bash, Glob, Grep
 model: opus
+effort: xhigh
 ---
 
 # tdd-author