Hotfix Roomote 1.8.2: restore Slack channel history reads

[roomote-roomote]

Summary

Backports the complete fix from #2681 onto the published v1.8.1 production baseline and generates the 1.8.2 patch release.

Source and prerequisites

• Source PR: [Fix] Slack channel history reads fail in busy channels #2681
• Source PR head: 7aa6df38db34308b0a47127f12c9b6981c51ee9c
• Merged squash commit: c463f9d7ccd1eadc74c6e28fae534433eccdba10
• Production base: dbd93ed7a709a0119e3c7254811fda1c02223b99 (v1.8.1)
• Hotfix head: acd130a728fcecdf291b32210fc93047bb1c099e
• Required prerequisites: none. The source patch applies cleanly and patch-identically to main.
• Schema/rollout safety: no schema, migration, or coordinated rollout changes; the N-1 rollback contract is unaffected.

Version and exact scope

• Previous version: 1.8.1
• Hotfix version: 1.8.2
• Runtime/test scope: packages/slack/src/slack-notifier.ts, its focused tests, and the API Slack channel lookup handler plus its focused test.
• Generated release artifacts: root package.json and CHANGELOG.md.
• No workspace package versions, docs, unrelated develop changes, or pending develop changesets are included.

The fix passes the caller's oldest bound to Slack, retains a capped best-effort lookback for older thread roots with recent replies, and surfaces the underlying Slack error instead of a misleading generic failure.

Validation

• The backport patch ID matches the merged source commit.
• pnpm run version generated 1.8.2, consumed the hotfix changeset, and left only .changeset/README.md and .changeset/config.json on the hotfix tree.
• git diff --check origin/main...HEAD passed.
• Ordinary pre-push gate passed: oxlint, residual lint, check-types:fast, and Knip.
• Source PR [Fix] Slack channel history reads fail in busy channels #2681 passed CI, CodeQL, and Roomote code review before merge.
• Hotfix-target CI and review must pass independently on this PR head before merge.

Live check

A real busy-channel Slack read is applicable, but no safe production-like Slack workspace/channel is connected to this sandbox. The request construction, bounded pagination, best-effort lookback failure, page cap, and API error propagation are covered deterministically by the backported tests. Treat the missing live check as an explicit release risk requiring maintainer acceptance or external verification before merge.

Release order

This PR must be merged into main with a merge commit, never squash or rebase. Do not merge companion PR #2683 until all of these are verified:

1. Tag Product Release succeeds for the production merge.
2. Annotated v1.8.2 resolves to the released tree.
3. Publish GHCR Images succeeds for app, worker, and gbrain immutable tags.
4. The GitHub Release for v1.8.2 is published.

Companion PR: #2683

The existing unshipped v1.9.0 Promote PR #2625 remains separate and must not be merged out of release order.

Rollback

Before tag publication, revert the hotfix merge commit if operationally safe. After publication, redeploy the previous immutable v1.8.1 images; never move, delete, or reuse a published tag. No data or schema cleanup is required.

[roomote-community]

No code issues found. See task

_{Reviewed acd130a}