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/spec-reviewer.md

@@ -61,7 +61,7 @@ your spec for this review.
 
 | Field | Content |
 |-------|---------|
-| `task_text_path` | Absolute path (typically `/tmp/iter-<iter_id>/task-<N>.md`) — same file the implementer received. Read this file as your first action. |
+| `task_text_path` | The same file the implementer received — see the authoritative `task_text_path` definition in `agents/implementer.md`. Read this file as your first action. |
 | `diff_command` | The shell command that produces the diff to review. Default and almost-always value: `git diff HEAD` (the working tree's full unstaged diff, accumulated across all tasks of this iter so far). Earlier tasks' changes are visible in this diff because no per-task commits are made — that is intentional; focus on the files the current task block claims it touches |
 | `status_from_implementer` | `DONE` or `DONE_WITH_CONCERNS` — if `DONE_WITH_CONCERNS`, the concerns are quoted so you can decide if they affect spec compliance |