refactor: single-source carrier/handoff contracts across skills

Apply the debug-skill pattern (commits 6410732..de42974) to the
remaining skills: the agent file is the single source for each
interface field's semantics; the SKILL.md copy is reduced to field
names plus a pointer, marking the agent's contract table
authoritative. Because SKILL.md loads into orchestrator context and
agents/*.md into the subagent's fresh system prompt with no
transclusion, duplicated field tables drift apart over time.

- brainstorm: carrier (spec_path/iteration_scope) -> grounding-check;
  the absolute-path requirement now lives only in the agent.
- planner: carrier (spec_path/iteration_scope/focus_hint) ->
  plan-recon, including the mandatory/optional markers and the
  BLOCKED-on-missing rule SKILL.md had omitted.
- fieldtest: carrier + produced fields -> fieldtester; the skill-level
  `status` roll-up (clean/friction_found/bugs_found/infra_blocked),
  which is not part of the agent's run-status protocol, stays defined
  in SKILL.md only.
- docwriter: carrier + produced fields -> docwriter agent.
- implement: carrier (iter_id scratch-dir/stats/not-a-branch
  semantics) -> implement-orchestrator; per-task sub-status vocabulary
  moved into the orchestrator-agent (it runs the loop in a fresh
  context and could not read SKILL.md at runtime, yet referenced "the
  sub-status table" by name); task_text_path single-sourced in
  implementer with spec-reviewer cross-referencing.

audit was already the reference implementation (pointers, no restated
contracts) and is unchanged.

closes #2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

brainstorm/SKILL.md

@@ -272,12 +272,11 @@ codebase (compiler, type-checker, runtime, schema, API,
 whatever the project owns), and for each one searches the
 workspace for a currently-green test that ratifies it.
 
-Dispatch carrier:
-
-| Field | Content |
-|-------|---------|
-| `spec_path` | path to the spec just written in Step 6 |
-| `iteration_scope` | which sections cover the imminent first iteration (verbatim from the brainstorm session) |
+Dispatch carrier — the fields `spec_path` and `iteration_scope`,
+defined authoritatively under **Carrier contract** in
+`agents/grounding-check.md` (note `spec_path` is an *absolute*
+path). That table is the single source for those fields; it is
+deliberately not restated here, so the two files cannot drift.
 
 The agent returns a `PASS` / `BLOCK` / `INFRA_ERROR` report.
 Orchestrator interpretation:

brainstorm/agents/grounding-check.md

@@ -54,6 +54,9 @@ section, not the whole spec.
 
 ## Carrier contract — what the controller hands you
 
+This table is the authoritative definition of the carrier fields;
+the dispatching skill references it rather than restating it.
+
 | Field | Content |
 |-------|---------|
 | `spec_path` | absolute path to the spec draft you are checking |