Add Block Kit support to message edits

[delaneyb]

Summary

Adds agent-slack message edit --blocks <path> so existing Slack messages can be updated with raw Block Kit blocks, matching the existing message send --blocks support.

What changed

• Adds a --blocks <path> option to message edit.
• Reuses the existing Block Kit JSON validation helper.
• Passes blocks through to Slack chat.update for both permalink targets and channel + timestamp targets.
• Documents the new edit option in the README, packaged skill, and command reference.

Tests

• Normal text edit remains text-only.
• Block edit by message URL.
• Block edit by channel + --ts.
• Malformed block JSON.
• Non-array block JSON.

Prepared by GPT-5.5 Medium using a custom sub-agent review process.

This change was reviewed by 5 sub-agents:

• 1 decomposition reviewer.
• 3 implementation/integration reviewers covering scope, CLI compatibility, test matrix, and local fork packaging.
• 1 clean external maintainer-style reviewer focused on upstream acceptance risk.

[nwparker]

✅ Taken over, verified, and merged into main via #121 (commit 80f0e8d).

Branch takeover/pr-92. Conflict in editMessage (main now auto-converts text→rich-text) resolved by mirroring the send path exactly: --blocks wins, else auto-convert. Also fixed a duplicate test import and dropped the now-deleted commands.md.

Verified:

• typecheck clean · 345 tests / 0 fail · lint 0 errors · build OK
• message edit --help shows --blocks; malformed JSON → --blocks: failed to parse JSON…; non-array → --blocks: expected a JSON array of Block Kit blocks, got object

Your commit 2e93bff is preserved. Thanks @delaneyb! Closing as merged.