feat(forum): agents hear and answer forum posts and comments

[BachirSaaS]

Summary

Forum channels were invisible to the agent surface, two ways:

1. Agents never heard forum activity. buzz-acp's Mentions subscription only carried kinds 9 / approval / reminder, so a forum post or comment that p-tagged an agent never started a turn — the mention was silently dropped ([Bug] Forum-post @mentions never wake agents — only kind 9 chat messages reach the agent harness #2850, buzz-acp: subscribe=mentions never receives forum-post mentions (kinds 45001/45003) #5268, buzz-acp does not start agent turns for p-tagged mentions in forum thread replies #4482, Forum thread mentions never reach managed agents #5249).
2. Agent replies vanished. When an agent did reply via buzz messages send, the CLI published kind 9, which the forum view never renders: the reply disappeared from the thread ([Bug] Agent forum replies publish as kind 9 and disappear from the forum thread #3828), and bare forum-channel sends were silently invisible (buzz messages send defaults to an invisible stream event for forum roots #5075).

Read side (buzz-acp)

• Forum post (45001) and comment (45003) kinds join the Mentions default subscription via one shared default_mention_kinds() helper, replacing the three divergent copies (startup rules, per-channel filters, dynamic channel filters).
• No dispatch changes needed: the author gate, mention filter, and queue are kind-agnostic, and forum comments carry NIP-10 thread tags, so the existing --reply-to reply instruction already fires for them.

Write side (buzz-cli)

• buzz messages send without an explicit --kind resolves the channel's kind:39000 metadata; in a forum channel a bare send becomes a post (45001) and a --reply-to send becomes a comment (45003).
• The routing lookup fails open to stream behavior on any error, so it can never block delivery. Agents following the existing --reply-to instruction now land as visible forum comments with no prompt changes.
• An explicit --kind stays authoritative.

Testing

• cargo test -p buzz-acp --lib — new asserts pin forum kinds in test_mentions_mode_default_kinds + new test_dynamic_mentions_mode_default_kinds (963 pass; the 2 lazy_pool failures also fail on clean origin/main).
• cargo test -p buzz-cli --lib messages:: — new channel_type_from_metadata_reads_t_tag; the two palette-RTT tests updated for the one routing lookup (36 pass).

Fixes #2850
Fixes #5268
Fixes #4482
Fixes #5249
Fixes #5075
Fixes #3828

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is d01e5f82058463709a22e93bb4cd795da5f53e10...962d5b7d4854fd040a425bf3718601367cf6d7f6.
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[BachirSaaS]

Closing — we'll maintain these fixes in our fork instead. Thanks for the look.