Slack dispatcher should support updating existing messages

[gugu91]

Problem

The Slack dispatcher currently does not expose a working action for updating existing Slack messages. Maintainer request: "we cannot update slack messages in the dispatcher, and fix it."

Expected

The compact Slack dispatcher should expose a discoverable update action for existing messages, with action-level guardrails and schema help. Required identifiers should include the Slack channel and message timestamp, plus updated text and/or blocks as appropriate.

Scope

• Add or restore dispatcher action support for chat.update.
• Preserve token-efficient progressive discovery: compact defaults, action-specific schema via dispatcher help.
• Use action-level guardrail naming consistent with other dispatcher actions.
• Add mocked tests for help/schema and update dispatch behavior.
• Update docs if the dispatcher action catalog is documented.

Notes

Filed from maintainer-priority Slack request in thread 1777980851.178719.