Preserve rich formatting on message edits

[AmethystLiang]

Replacement for #93 because maintainer pushes to the contributor fork were rejected with 403 despite maintainerCanModify=true.

This includes the original PR plus review fixes:

• Keep inline-only edits text-only so normal edited-marker behavior is preserved where blocks are not needed.
• Preserve Slack channel/usergroup/broadcast tokens in rich-text conversion.
• Preserve supported manual links, including http(s) and mailto.
• Keep non-URL angle-bracket tokens literal.
• Sync README and bundled agent-slack skill docs.

Validation:

• bun test
• bun run typecheck
• bun run format:check
• bun run lint
• Independent xhigh Codex review: no blocking findings.