Classify failed accepts as errors, and fix parallel lane race/all misuse

Two of the three new findings, plus the bug that chasing them exposed in my own
earlier fix. The third is mitigated rather than broken; details below.

Failed accepts reported success:
live/completion.mjs only classifies a result as `error` when it carries
`mode: 'error'`. Everything else unhandled falls through to `agent_done` with an
ok ack, which is deliberate for the documented fallback paths (two tests pin it)
but wrong for a real failure. So `accept_receipt_conflict` reported success, and
reference/live.md's `handled: false` without `mode` bullet told the agent to
"read file, find markers, edit" — hand-applying a second accept on top of the one
the receipt already recorded.

The same hole swallowed `source_locked`, which is mine: the earlier commit made
lock contention return clean JSON so the agent could retry, but the classifier
turned that failure into agent_done/ok, so the accept was dequeued and silently
lost. Mark genuine failures with `mode: 'error'` through one `operationFailure`
helper, and give live.md a `mode: "error"` bullet with per-error guidance: retry
the same command on `source_locked`, never hand-edit, and on a receipt conflict
report what the session actually resolved to. The deliberate fallback and
markers-not-found handoffs stay untouched.

parallel-compact lane orchestration:
`Promise.race` settles on the first *settlement*, so one lane failing fast
rejected the whole first-variant step while two lanes were still on their way to
succeeding. `Promise.any` now takes the first success and only a total wipeout is
fatal, reporting every lane's reason. The tail step's `Promise.all` surfaced a
raw lane error non-deterministically; `Promise.allSettled` now reports how many
lanes failed and why. Added a `requestImpl` seam so lane orchestration is
testable without a provider key.

Not a defect: the browser releasing Accept before the source write. That is the
intended optimistic design, and it is safe because poll-lanes ranks accept at
priority 0 against generate at 2, so a queued accept is always leased before a
generate the user queues afterwards, even if the generate arrived first. Its
source write lands inside the poll script before the next generate preflights.
That invariant is load-bearing and had no tests at all; poll-lanes.mjs now has a
suite covering it plus lease and type filtering.

Prepared with AI assistance under maintainer direction.

Co-Authored-By: Claude <noreply@anthropic.com>

skill/reference/live.md

@@ -508,6 +508,10 @@ Event: `{id, variantId, _acceptResult, _completionAck}`. The poll script already
 - `_acceptResult.handled: true` and `carbonize: false`: nothing to do. Poll again.
 - `_acceptResult.handled: true` and `carbonize: true`: post-accept cleanup is required, but it must not stall Codex's control lane. See "Required after accept (carbonize)" below. The `event._acceptResult.todo` field, `_completionAck.requiresComplete`, and stderr banner all point at this required follow-up; none are decorative.
 - `_acceptResult.handled: false, mode: "fallback"`: the session lived in a generated file and the script refused to persist there. You've already written the accepted variant into true source during Handle fallback Step 3; just clean up the temporary wrapper in the served file if any, and poll again.
+- `_acceptResult.handled: false, mode: "error"`: the operation genuinely failed. **Do not hand-edit the file**; the source was not touched and editing it yourself would either double-apply or race whoever holds it.
+  - `error: "source_locked"`: a generation publish holds the file. Run the same `live-accept.mjs` command again; it is idempotent and will succeed once the publisher releases. Do not poll past it.
+  - `error: "accept_receipt_conflict"`: this session already resolved as `priorOperation` (on `priorVariantId` for an accept), so the request contradicts durable truth. Do not edit. Run `live-status.mjs` and tell the user what the session actually resolved to.
+  - anything else: report the error briefly and run `live-status.mjs` before continuing.
 - `_acceptResult.handled: false` without `mode`: manual cleanup: read file, find markers, edit.
 
 ### Required after accept (carbonize)