slack: promote legacy-attachment content to message body text

[Soony123]

Problem

Notification bots — GitHub's Slack app among them — post messages whose text is empty, with the content living only in legacy attachments (fallback/pretext/title/text). Every ingestion path read m.text alone, so these messages were invisible end to end:

• the thread-reply dispatch dropped them as empty unprompted input,
• the ambient judge filtered them out of its batch ((m.text ?? "").trim()), so an action-mode bot policy or standing orders aimed at such a bot could never fire,
• the conversation view rendered them as blank lines.

Change

messageBodyText (in message-gating.ts, exported via slack/lib.ts) returns the text verbatim when present and otherwise composes the body from attachment content, preferring the richer pretext/title/text fields over the fallback summary. Applied at every ingestion site: both events.ts dispatches, mirror.ts surface ingestion, and conversation-view.ts rendering (including thread openers).

Verification

• Unit tests added in test/slack-message-gating.test.ts (verbatim text, fallback composition, multi-attachment join, field preference).
• Verified against a live instance: a label-filtered GitHub PR notification in a channel with standing orders now reaches the ambient judge with its body visible and produces an engagement (the agent reviewed the PR and commented). Before the change the same notification produced no ambient judgment at all — before/after visible in the surface cache: pre-change rows for such notifications have empty text, post-change rows carry the attachment body.
• tsc --noEmit, eslint on touched files, and the slack message-gating/conversation/mirror test files all pass.

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