fix(workflows): fail fast on a malformed args carrier instead of misreporting it

Both workflow scripts destructured object-form `args` behind `args || {}`
only. A named-workflow invocation can deliver `args` as a string; a
non-empty string is truthy, so every carrier field silently read
`undefined` and the failure surfaced far from the cause: implement-loop
dispatched its plan-index agent against "Read the plan at undefined" and
stopped only via a mislabelled NEEDS_CONTEXT after burning agent calls;
compiler-driven-edit briefed its edit agent with "EDIT: undefined" and
mapped the agent's refusal onto the straddle-rule BOUNCE to specify —
a mechanics failure dressed as a design finding.

Guards now run before any agent dispatch, mirroring the existing
malformed-input pattern (the task_range guard):

- A string carrier is first JSON-parsed: a JSON-serialized object
  carrier (a stringifying caller or substrate layer that still authored
  the documented object form) is accepted; free text does not parse to
  an object and is rejected.
- A carrier that is not an object (string, number, boolean, array) is a
  distinct infra BLOCKED naming the received type (compiler-driven-edit:
  kind bad-carrier) — never a bounce, never NEEDS_CONTEXT.
- Missing or blank required fields fail fast by name, per mode:
  standard iter_id+plan_path, mini iter_id+red_test_path+cause_summary,
  compiler-driven-edit edit_description+def_site. implement-loop also
  rejects an unknown mode, since the required-field set keys off it.

String-form args are NOT a documented interface — the only invocation
examples in the plugin are object-form (implement/SKILL.md); the issue's
free-text-passthrough fix part was dropped on that ground (triage
verification on the issue). implement/SKILL.md now also documents the
infra early-exit end-report shape (minimal {status, iter_id, reason},
no BLOCKED.md, no blocked_detail) in the Iron Law exceptions, Step 2,
and Step 4's no-file special case.

Verified: both scripts pass node --check (async-wrapped, as the Workflow
substrate runs them) and a 17-case stub-agent harness — malformed
carriers block with zero agent dispatches; valid and JSON-serialized
object carriers reach the first agent unchanged.

closes #24

implement/SKILL.md

@@ -74,7 +74,9 @@ Its executor is **observe-then-bounce**, shipped as
    would otherwise read as DONE. A clean working tree after the edit
    phase is a no-op, not a success → distinct `BLOCKED` (no-op-edit),
    never DONE. Positive evidence (tree differs from HEAD) is required,
-   checked both at the edit phase and independently at verify.
+   checked both at the edit phase and independently at verify. (A
+   malformed or missing carrier is likewise a distinct `BLOCKED`
+   (bad-carrier) before any agent runs — never a bounce.)
 4. **Bounce** otherwise — the change was not purely behaviour-preserving
    after all, so route up per the straddle rule: a site that forces a
    design decision → `specify`; a site that turns out to **encode new
@@ -102,7 +104,7 @@ WITHIN THIS LOOP, MAIN HEAD IS SACROSANCT — NO RESET, NO REVERT; THE LOOP ONLY
 PER-TASK PHASES ARE SEPARATE AGENT CALLS IN THE WORKFLOW SCRIPT — implementer, then spec-compliance, then quality. SPEC COMPLIANCE IS GATED BEFORE QUALITY.
 TASKS RUN SEQUENTIALLY; A BLOCKED TASK STOPS THE LOOP — NO SKIP-AHEAD (TASK ORDERING DEPENDENCIES ARE UNKNOWN).
 NEVER PUSH PAST `BLOCKED` BY HAND.
-ON `PARTIAL` OR `BLOCKED`, THE SCRIPT WRITES `BLOCKED.md` AT THE REPO ROOT — UNCOMMITTED BY CONVENTION. ON `DONE`, NO SEPARATE FILE. (ONE EXCEPTION: A NO-OP ITERATION IS `BLOCKED` WITH NO `BLOCKED.md` — ITS TREE IS ALREADY CLEAN, SO THE STATUS + REASON RIDE THE END-REPORT AND THERE IS NOTHING TO CLEAN UP.)
+ON `PARTIAL` OR `BLOCKED`, THE SCRIPT WRITES `BLOCKED.md` AT THE REPO ROOT — UNCOMMITTED BY CONVENTION. ON `DONE`, NO SEPARATE FILE. (EXCEPTIONS THAT WRITE NO `BLOCKED.md`: A NO-OP ITERATION — ITS TREE IS ALREADY CLEAN — AND ANY PRE-LOOP EARLY EXIT (MALFORMED CARRIER, DIRTY-TREE PREFLIGHT, PLAN-EXTRACT GUARD): NOTHING RAN AND NOTHING WAS WRITTEN, SO THE STATUS + `infra:`-PREFIXED REASON RIDE THE END-REPORT AND THERE IS NOTHING TO CLEAN UP OR EXPLAIN.)
 `DONE` REQUIRES POSITIVE EVIDENCE THAT AN EDIT LANDED — A NO-OP ITERATION (`git status --porcelain` EMPTY, untracked files INCLUDED) IS NEVER `DONE`, IT IS `BLOCKED` (the issue #11 vacuous-green shape, guarded on the main path). THE VERDICT IS THE GIT TREE COUNT, NEVER A SELF-REPORT.
 IN MINI MODE THE RED->GREEN IS OBSERVED, NOT ASSERTED: AN INDEPENDENT VERIFY RE-RUNS THE HANDED-OFF RED TEST AND THE SUITE; A STILL-RED TEST OR A REGRESSION IS `BLOCKED` (route back to debug, RED-first).
 THE COMPILER-DRIVEN ARM COMMITS ONLY ON CLEAN BUILD + SUITE GREEN UNCHANGED; ELSE IT BOUNCES (specify for a design hole, tdd for discovered new behaviour, debug for a regression).
@@ -197,7 +199,10 @@ per-task summaries, concerns, E2E fixtures, the stats path, and — on
 PARTIAL/BLOCKED — the blocked detail. Per-task chatter stayed inside
 the workflow's agent contexts and never reached the orchestrator. Read
 the end-report; it is the per-task summary you build the commit body
-from.
+from. An **infra early-exit** (malformed or missing carrier, dirty-tree
+preflight, a plan-extract guard) returns a minimal report instead:
+`status: BLOCKED` plus an `infra:`-prefixed `reason`, no `blocked_detail`
+and none of the per-task fields — nothing ran.
 
 ### Step 3 — Orchestrator inspect + commit step (on DONE)
 
@@ -243,6 +248,11 @@ tree:
    already clean. The diagnostic is in the end-report's `blocked_detail`, not
    a file. Nothing to clean up or stash; re-plan or re-run from the same
    clean tree.
+   The same no-file shape occurs for an **infra early-exit** (`reason`
+   prefixed `infra:` — malformed/missing carrier, dirty preflight tree, a
+   plan-extract guard): no `BLOCKED.md`, no `blocked_detail`, and the tree
+   exactly as the loop found it. Fix the invocation (or clean the tree) and
+   re-invoke.
 1. Read `BLOCKED.md` — `## What did not` names the failure mode and the
    blocked task's reason. Read `git diff` to see what was attempted.
 2. Decide: