fix(buzz): adopt channels joined during runtime

[dplush]

Fixes #75107.

When BUZZ_CHANNELS is unset, a membership event now discovers newly joined named community channels, seeds their high-water mark, and subscribes to them without requiring a gateway restart. Explicit channel lists remain authoritative; DM-shaped fallback conversations continue through the existing DM path.

Verification:

• Red regression test committed first (69d53af).
• scripts/run_tests.sh tests/gateway/test_buzz_adapter.py tests/gateway/test_buzz_websocket.py — 30 passed.
• uv run --extra dev --frozen ruff check plugins/platforms/buzz/adapter.py tests/gateway/test_buzz_websocket.py — passed.
• git diff --check origin/main — clean.
• reviewer-claude-review diff origin/main --narrow — no blockers.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #74823: both open PRs implement runtime adoption/subscription of newly joined Buzz channels for #75107. #76156 remains related because it adds distinct addressing and reconciliation scope.

[teknium1]

Thanks for the focused regression coverage. Current main reproduces the reported premise: plugins/platforms/buzz/adapter.py:801-813 handles membership events by rediscovering only DMs, while plugins/platforms/buzz/adapter.py:949-963 deliberately excludes named community channels from that fallback.

Problems

• The added _discover_joined_channels() queries unfiltered channels list. The automatic-mode contract is “all joined channels” (website/docs/user-guide/messaging/buzz.md:51), but this PR does not prove that this CLI query is member-scoped. The linked duplicate fix(buzz): discover newly joined channels dynamically #74823 instead uses channels list --member and adds an unjoined-channel exclusion test. This draft's new tests fixture only the newly joined channel.

Suggested changes

• Use the member-scoped listing (or establish the unfiltered CLI contract) and add a regression that a listed-but-unjoined channel is not adopted.
• Compare with fix(buzz): discover newly joined channels dynamically #74823 for salvage; it covers the related member-filtered startup and membership-race behavior.

Automated hermes-sweeper review.