Add a per-platform `cron_continuable_surface` extra key
(`thread` default | `in_channel`) so a continuable cron job can deliver
FLAT into a Slack channel — no dedicated thread — and still be
replied-to. In `in_channel` mode the scheduler skips the thread-open
branch (leaves `thread_id=None`); the shipped origin-mirror then seeds
the `(slack, chat_id, None)` shared-channel session — the same bucket
`reply_in_thread: false` routes inbound channel replies to — so a plain
channel reply continues the job in context.
Design: specs/cron-inchannel-continuable (D1–D7, F5). Model B
(shared-channel session), NOT anchoring to the delivery `ts` — on Slack
replying to a specific message IS threading, so a `ts` anchor would only
relocate the thread, never deliver true threadless continuable.
- gateway/platforms/base.py: `supports_inchannel_continuable` capability
flag (default False → unsupported platforms fail SAFE to `thread`).
- plugins/platforms/slack/adapter.py: flag=True; `_cron_continuable_surface()`
resolver (coerces to the two-value enum); `_warn_if_inchannel_without_flat_reply`
connect-time warning (D5: warn, not hard-require — the misconfig fails safe).
- gateway/config.py: shared-key bridge line (top-level OR nested config).
- cron/scheduler.py: read the key generically from platform config, gate
the `in_channel` branch on the adapter capability flag, skip thread-open.
No new seed function (reuses the existing mirror — G6).
Pairing (docs): `in_channel` + `reply_in_thread: false` +
`require_mention: false` (or a free-response channel). Missing
`reply_in_thread: false` fails safe to a threaded continuation.
Gateway-side config flag — `/restart` to apply; NO Slack app reinstall.
Tests (from inside the worktree, PYTHONPATH=$PWD):
- +6 cron scheduler tests (in_channel skips thread-open; seeds flat
channel session with thread_id=None; thread-mode regression;
fail-safe on unsupported platform; value coercion). Prove-fail:
removing the `and not in_channel_surface` guard turns the two
load-bearing tests RED; restore → GREEN.
- +10 slack resolver/capability/warning tests; +2 config-bridge tests.
- tests/manual/cron_inchannel_e2e.py: offline E2E driving BOTH real
legs (delivery seed + inbound reply keying) → both converge on
(slack, C, None).
- No regressions: test_slack.py 216 passed alone; broader sweep green
(4 pre-existing cross-file-ordering failures reproduce identically on
pristine origin/main).
Docs: cron.md + slack.md + zh-Hans mirrors of both.
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