Widen agent-automerge to include operant-cron/* branches

## Problem

`agent-automerge.yml` in Portico (and by extension the same class of workflow in operant-studio if it exists) filters strictly on `head.ref` starting with `agent/POR-`. Any operator-authored or cron-authored PR on an `operant-cron/*` branch is silently skipped by automerge and requires a manual click to land.

## Evidence

All six infrastructure PRs shipped tonight required manual `gh pr merge` because their branches were `operant-cron/*` or `operant-cron/OPER-*`:
- PR #47 (OPER-6): manual merge
- PR #48 (OPER-6.1): manual merge
- PR #49 (OPER-7A): manual merge (+ size-override)
- PR #50 (OPER-11A): manual merge (+ size-override)
- PR #414 (auto-ready IGNORE self, Portico): required `--auto` flag after PR opened — this is the ticket that surfaced it

Each adds ~5-15 min click-latency to your day. Multiply by tenor of infra work and the operator becomes the bottleneck.

## Success criteria

1. `agent-automerge.yml` (Portico) queues automerge for PRs whose `head.ref` starts with either `agent/POR-` OR `operant-cron/`.
2. Same widening in any equivalent workflow in `Operant-Health/operant-studio` (search for automerge/auto-merge workflow files).
3. All existing safety gates preserved:
   - not draft
   - title does not start with `[BLOCKED]`
   - size/XL requires `size-override` label
   - required checks (Lint, Typecheck, Unit Test, E2E) must be green before squash
4. `agent-auto-ready.yml` companion widening — the ready-flip workflow filters on the same `agent/POR-*` prefix. If we widen automerge without widening ready-flip, `operant-cron/*` PRs will merge from ready-state but never get flipped from draft (moot for me since I open non-drafts, but consistency matters).
5. Document the branch-name convention in the workflow file header comment so future contributors know which prefixes trigger automation.

## Suggested implementation

Replace the current filter:
```
startsWith(github.event.pull_request.head.ref, 'agent/POR-')
```
with either:
```
(startsWith(head.ref, 'agent/POR-') || startsWith(head.ref, 'operant-cron/'))
```
or introduce a shared regex constant. Prefer the explicit-OR form for grep-ability.

Apply the same change in `agent-auto-ready.yml`, the `find eligible agent PRs` jq filter, and any sweeper cron logic.

## Repo(s)

- `Operant-Health/portico` (`.github/workflows/agent-automerge.yml`, `agent-auto-ready.yml`, sweeper cron if separate)
- `Operant-Health/operant-studio` (any equivalent automerge workflow — check before assuming absence)

## Out of scope

- Widening to *any* branch prefix (would let non-operator PRs auto-merge — no)
- Changing required-check list (that's OPER-14A / worker-reaper territory)
- Adding new labels or approval gates

## Priority

1 (removes recurring 5-15 min of operator click-latency per infra PR; unblocks true 24/7 autonomous pipeline).

## Related

- PR #414 (Portico auto-ready IGNORE fix) surfaced this — it needed `gh pr merge --auto` because it fell outside the automerge filter
- OPER-11A (branch-ensure at claim-time) mints `justin/<id>-*` branches for dispatched work — those match neither prefix, so this is only for cron/operator-authored infra PRs
- Complements 2x-throughput plan Lever 2a and standing rule "pipeline should be moving at all times"