feat(message send): add --blocks flag for raw Block Kit payloads

[thehesiod]

Summary

Adds a --blocks <path> option to message send that reads a JSON array of Block Kit blocks from a file (or - for stdin) and passes it to chat.postMessage as-is, bypassing the built-in markdown-to-rich-text conversion.

This unlocks structured layouts that the markdown converter can't express — header, divider, context, section (with accessory), and especially the table block introduced by Slack in August 2025. User tokens already carry the chat:write scope needed; no new auth is required.

Example

cat > blocks.json <<'JSON'
[
  { "type": "header", "text": { "type": "plain_text", "text": "Weekly digest" } },
  {
    "type": "table",
    "rows": [
      [{ "type": "raw_text", "text": "Name" }, { "type": "raw_text", "text": "Why" }],
      [{ "type": "raw_text", "text": "Caveman MCP" }, { "type": "raw_text", "text": "~80% token cut" }]
    ]
  }
]
JSON
agent-slack message send "#alerts-staging" --blocks blocks.json

Behavior

• --blocks and --attach are mutually exclusive (uploads take a separate code path that doesn't carry blocks)
• Positional <text> remains optional when --blocks is set, but if provided it is sent as the message text fallback used for notifications and unfurls
• Malformed JSON and non-array payloads produce clear CLI errors
• When --blocks is set, the automatic markdown→rich_text conversion is skipped

Files

• src/cli/message-command.ts — add --blocks option + mutual-exclusion check
• src/cli/message-actions.ts — loadBlocksFromPath() reads/parses/validates; sendMessage() prefers loaded blocks over text-derived ones
• README.md, skills/agent-slack/SKILL.md, skills/agent-slack/references/commands.md — document the flag with a table-block example
• test/message-send.test.ts — 4 new cases (happy path, non-array JSON, malformed JSON, precedence over text)

Test plan

• bun test — all 185 tests pass (4 new)
• bun run typecheck — clean
• bun run lint — no new warnings (5 pre-existing line-count warnings unchanged)
• bun run format:check — clean
• Built locally with bun run build and sent a real message to a DM with a header + table block; table rendered natively in the Slack client

[AmethystLiang]

will review and get this in today

[AmethystLiang]

Hi @thehesiod — thanks for this! I merged a continuation of your work as #89 (released in v0.9.3) since I couldn't push directly to your fork to resolve the merge conflict with main.

The squash commit on main credits you, and I added one small follow-up: loadBlocksFromPath now validates each array element is an object so non-object entries fail with a clear local error rather than a Slack-side rejection. Closing this in favor of the merged PR.

[thehesiod]

thanks to @claude :)