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>

implement/agents/implement-orchestrator.md

@@ -49,7 +49,10 @@ Additionally, every dispatch:
 
 ## Carrier contract
 
-You receive from the outer orchestrator:
+You receive from the outer orchestrator the following fields. This
+table is the authoritative definition of each field's semantics;
+the dispatching skill shows the call shape but references this table
+rather than restating the semantics.
 
 | Field | Content |
 |-------|---------|
@@ -147,6 +150,34 @@ them as you switch roles:
 - Spec-compliance check reference: `spec-reviewer.md`
 - Quality check reference: `quality-reviewer.md`
 
+##### Per-task sub-status vocabulary (authoritative)
+
+The phases below produce these sub-status values. They are
+*internal* to you — they describe the state of an inline
+role-phase, not the status of a separate subagent, and are not
+reported upstream. This table is the authoritative definition of
+the vocabulary and the re-loop rule; the per-phase prose that
+follows applies it, and the dispatching skill names it but
+references this table. The vocabulary maps 1:1 to the phase
+reference files listed above.
+
+| Sub-status | Action |
+|------------|--------|
+| `DONE` | next phase / next task |
+| `DONE_WITH_CONCERNS` | accumulate concern, next phase |
+| `NEEDS_CONTEXT` (1st–2nd) | re-attempt the phase with expanded context |
+| `NEEDS_CONTEXT` (3rd) | stop → `BLOCKED` to orchestrator, reason `context-exhausted` |
+| `non_compliant` / `changes_requested` (1st–2nd) | switch back to implementer mindset, repair with the check's report as repair brief, then re-run the check phase |
+| `non_compliant` / `changes_requested` (3rd) | stop → `BLOCKED` to orchestrator, reason `review-loop-exhausted` |
+| `BLOCKED` (implementer phase) | stop → `BLOCKED` to orchestrator, reason verbatim |
+| `unclear` (spec-compliance phase) | stop → `BLOCKED` to orchestrator, reason `spec-ambiguous` |
+| Tool / infra error | stop → `BLOCKED` to orchestrator, reason `infra` + raw error |
+
+Re-loop limit: 2 retries per failure-mode per task. The 3rd is
+`BLOCKED` to the orchestrator. `skip task K, continue` is
+intentionally NOT a mode — tasks have implicit ordering
+dependencies and you do not know the dependency graph.
+
 #### 2.1 — Implementer phase (inline)
 
 Read `/tmp/iter-<iter_id>/task-K.md`. Adopt the implementer