feat(slack): render agent bodies as native markdown blocks

[yzxcj797]

Summary

Implements the fix shape from #651 verbatim: every outbound agent body renders once into { text, blocks } — text stays the existing toSlackMrkdwn fallback (notifications, mirrorSelfPost cache, older clients), and a single markdown block carries the raw Markdown, where lists, tables, fenced-code languages and CJK-flanked bold translate into native Slack elements instead of character-encoded approximations.

Implementation

• renderSlackBody(md) in mrkdwn.ts returns both renderings; slackMarkdownBlocks wraps the raw Markdown. The block is emitted only when the body fits SLACK_MARKDOWN_BLOCK_BUDGET = 11,500 — Slack's 12,000 budget applies to the translated result cumulatively, so over-budget bodies fall back to today's text-only path, keeping Markdown-safe chunking (fences, table rows, ordered-list continuity) off the critical path per the issue's note 4.
• All six agent-body sites render through it:
  • turn-handler.ts — the ack presenter and the turn reply (postReply already accepted blocks);
  • approvals.ts — both result paths; the non-remote card update resends the block (an update with no blocks clears them, per note 5), and postApprovalFollowup threads it; the agent-to-agent completion header is bolded ** for the block and stays * in the text fallback, since *x* is italic in Markdown (note 1);
  • deliveries.ts — both pollers; the channel poller's markdown block replaces its section-block body while the task-list section block survives alongside (per the "a section block sent alongside it survived" measurement), and the principal poller posts it directly.
• The text fallback path is byte-identical to today, so test/slack-mrkdwn.test.ts's existing expectations are unchanged.

Testing

• 4 new tests: the helper's contract (raw Markdown in the block, mrkdwn in text; over-budget and blank bodies fall back to text-only; the budget stays under the 12k cap) and a wiring contract asserting every one of the six sites threads the blocks. Differential: with the source changes stashed the suite fails to import; neighbors (slack-delivery, slack-approval-cards, slack-ack-emoji) are 72/72.
• Note 6's live-scenario assertion (test/live-slack asserting • lines) still applies to the text layer, which is unchanged; the native-list layer only appears through the block, which the live suite would need block-structure assertions for — left to that suite's maintainers since it requires a live workspace.

Fixes #651

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