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>

docwriter/SKILL.md

@@ -59,22 +59,17 @@ agent file governs the work.
 
 ## Handoff Contract
 
-`docwriter` consumes (from orchestrator):
+`docwriter` consumes (from orchestrator) the carrier fields
+`scope`, `warning_target`, and `priority_items`, defined
+authoritatively under **Carrier contract** in `agents/docwriter.md`.
 
-| Field | Content |
-|-------|---------|
-| `scope` | Component / package / crate / module name(s) to document, or `all` |
-| `warning_target` | Specific doc-tool warnings to clear, or `all` |
-| `priority_items` | Optional: items the orchestrator wants documented first |
+`docwriter` produces, for the orchestrator, the fields `status`,
+`files_touched`, `warnings_cleared`, and `findings`, defined
+authoritatively under **Output format** (with the `status` enum in
+the Status protocol) in `agents/docwriter.md`.
 
-`docwriter` produces:
-
-| Field | Content |
-|-------|---------|
-| `status` | `DONE` / `DONE_WITH_CONCERNS` / `NEEDS_CONTEXT` / `BLOCKED` |
-| `files_touched` | paths + which level (top-level / module-level / item-level) |
-| `warnings_cleared` | count, plus the doc-check status |
-| `findings` | items whose names or behaviour seemed confusing while documenting them (one line each, no prescriptions) |
+Both contracts are single-sourced in the agent file; they are
+deliberately not restated here, so the two files cannot drift.
 
 The orchestrator decides whether findings become a follow-up
 iteration; `docwriter` does not self-resolve.

docwriter/agents/docwriter.md

@@ -57,6 +57,9 @@ Additionally:
 
 ## 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 |
 |-------|---------|
 | `scope` | Component / package / crate / module name(s) to document, or `all` |
@@ -160,6 +163,18 @@ At most 200 words, structured:
   prescriptions. The orchestrator decides whether they become
   a follow-up.
 
+This section (together with the Status protocol above) is the
+authoritative definition of what the role produces; the
+dispatching skill references it rather than restating it. Named
+for the orchestrator handoff, the produced fields are:
+
+| Field | Content |
+|-------|---------|
+| `status` | `DONE` / `DONE_WITH_CONCERNS` / `NEEDS_CONTEXT` / `BLOCKED` (see Status protocol) |
+| `files_touched` | paths + which level (top-level / module-level / item-level) |
+| `warnings_cleared` | count, plus the doc-check status |
+| `findings` | items whose names or behaviour seemed confusing while documenting them (one line each, no prescriptions) |
+
 ## Common Rationalisations
 
 | Excuse | Reality |