feat: Add Slack transport

[victor-develop]

This PR backports the upstream-safe Slack transport and runtime work that was developed and validated in the long-lived ductor-slack fork.

Included here

• Slack transport, startup wiring, sender, ID mapping, transport registration, and capability plumbing
• Slack setup in the init wizard, config validation, and stale transport self-healing in ductor_bot.__main__
• Background, named-session, heartbeat, and bus delivery fixes so Slack replies preserve transport and thread targets
• Native Slack streaming with richer tool metadata plus improved thinking/reasoning presentation
• Duplicate suppression for the same Slack post arriving via both message and app_mention
• Upstream-safe docs/tests needed to support the Slack runtime path

Explicitly excluded

• fork-only coexistence/package rename changes (ductor_slack, ductor-slack, ~/.ductor-slack)
• fork-only deploy Docker image, packaging/docs, and app-manifest setup
• fork-only upstream release watcher workflow
• fork-only release/versioning changes and workspace-path isolation

Status

• merged with current origin/main
• PR merge state is clean
• the fork-only release watcher has been removed from this PR branch and remains only in the fork
• draft status is unchanged; this description is just bringing the scope/status up to date

[victor-develop]

Hi, I've tested in my own slack instance. Looking forward to review/comments or merge.

[PleasePrompto]

I reviewed this for v0.17.0 and I am not merging it yet. I found release/supply-chain blockers that need cleanup first:

• .github/workflows/upstream-release-watch.yml is still present and creates issues on a schedule, although the PR body says the fork-only watcher was removed.
• config.example.json is invalid JSON because the Slack section has a trailing comma.
• Fork-only ductor-slack strings remain in wizard/i18n text.
• Slack dependencies are added, but uv.lock is not updated.

I did not find obvious credential exfiltration in the inspected Slack runtime paths, but this is a large transport addition and needs a clean follow-up before upstream merge. Thanks for the work here.