fix(agentex): put the connect link where the user is actually looking

[michael-chou359]

What

The first real link offer in production was delivered correctly and reported as
never received. Both were true:

• chat.postMessage returned ok, and conversations.history confirmed the message
  sitting in the DM channel.
• Slack files bot conversations under Apps, not in the Direct messages list — so
  "I've DM'd you a link" pointed at the one place it wasn't.

The connect link now goes in the ephemeral as well as the DM.

before                                  after
ephemeral: "I've DM'd you a link —      ephemeral: "<Connect your SGP account>
            check your DMs"                         …also sent to <our DM>"
DM:        <connect link>               DM:        <connect link>   (unchanged)
clicks to connect: 2, if you find it    clicks to connect: 1

Why this doesn't weaken anything

An ephemeral has exactly the same audience as a DM: Slack renders it for one user,
keeps it out of channel history, and out of search. The exposure argument that made
this DM-only never applied to an ephemeral — so routing someone through a conversation
they can't find, to click a link we could have handed them directly, bought nothing.

The DM stays because ephemerals are transient: reload Slack before clicking and
it's gone, and the offer cooldown would then block a retry for an hour. So the
ephemeral carries the link plus a deep link to the DM as the durable copy.

The invariant, stated properly

This change moves the line, so the rule is now written down precisely:

The nonce is a bearer token. It may go anywhere exactly one person can see it
(the user's DM, an ephemeral addressed to them) and nowhere that lands in channel
history.

The test that asserted "never in a payload addressed to the origin channel" now
asserts "never in a chat.postMessage outside the user's own DM". That matters: the
old wording would have failed this change while the actual risk — a channel-visible
bearer token — was never touched by it. A test that encodes the implementation rather
than the property is one that blocks correct changes.

Also

• conversations.open moved above the send-cap check, since both branches now need
  the channel id. It's idempotent (returns the existing DM rather than creating one).
• Past the DM cap the user still gets the live link. The cap limits DMs, not what
  we're allowed to show the person in front of us.
• Deep links use slack.com/app_redirect, not a slack:// URI, which fails on the
  web client.

Testing

3 new, 3 rewritten. The rewrites are the interesting ones — they invert assertions
that encoded the old design:

Was	Now
nonce must not appear in the ephemeral	nonce must appear in the ephemeral (single-viewer)
nothing addressed to the origin channel carries it	nothing broadcast carries it
cap path posts only an ephemeral	cap path opens the DM first, still no second DM

New: the ephemeral points at the durable copy; the cap path hands over the live link;
an unreachable DM offers nothing at all (we can't link to a conversation that doesn't
exist).

93 in the gateway suite, 689 across unit, ruff clean.

Not fixed here

The nonce is still 10 minutes. That was sized assuming prompt discovery, which this
incident showed was optimistic — but with the link now inline in the ephemeral, the
find-it delay largely disappears, so I'd rather see whether it's still a problem than
change two things at once.

🤖 Generated with Claude Code

Greptile Summary

The PR makes Slack account-link offers immediately discoverable by including the single-user connection link in the ephemeral response while retaining the DM as a durable copy.

• Opens the user’s DM before evaluating the DM send cap so both response branches can construct a Slack deep link.
• Adds the live account-link URL and DM redirect to normal and capped ephemeral responses.
• Reworks unit tests around the single-viewer security invariant, capped sends, deep linking, and unreachable DMs.

Confidence Score: 5/5

The PR appears safe to merge, with the bearer nonce remaining confined to the user’s DM and explicitly user-targeted ephemeral messages.

The changed delivery flow preserves the single-viewer boundary, fails closed when a DM cannot be opened, and uses the same currently valid nonce for capped responses; no actionable changed-code defect remains.

Important Files Changed

Filename	Overview
agentex/src/domain/use_cases/slack_gateway_use_case.py	Adds the nonce URL and DM deep link to user-targeted ephemerals without introducing a channel-visible fallback or weakening the existing recipient boundary.
agentex/tests/unit/use_cases/test_slack_gateway_use_case.py	Updates the security invariant and adds focused coverage for ephemeral link delivery, durable-copy redirects, send-cap behavior, and DM-open failures.

_{Reviews (1): Last reviewed commit: "fix(agentex): put the connect link where..." | Re-trigger Greptile}

[michael-chou359]

Folded into #414 — same files, and the flag cut and this fix read better as one change.