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.