[StephenTangCook]

Summary

Adds an opt-in host-backed channel mention picker to the rich text editor. Typing # at a valid trigger position or choosing the new Channel toolbar action searches channels and inserts an exact Slack { type: "channel", channel_id } element.

• Extends MentionsConfig with searchChannels and getChannelName, plus the exported MentionChannel type.
• Preserves existing channel mentions in WYSIWYG mode, including supported bold, italic, strike, and underline styles.
• Keeps the @ and # suggestion plugins independent and available in the same editor.
• Exposes combobox/listbox state to assistive technology while focus remains in the editor.
• Falls back to structured mode for unsupported channel styles, code-block mentions, and malformed channel elements.

Why

ENG-6388 needs Message Blaster admins to tag Slack channels without editing raw Block Kit JSON. Search remains host-brokered so consumers control tenant and caller visibility.

Test plan

• pnpm typecheck
• pnpm lint
• pnpm test (723 tests)
• pnpm build
• Added Storybook coverage for seeded user and channel mentions.
• Verified typed and toolbar channel picks, keyboard selection, Escape behavior, exact Slack serialization, name fallback, no-config round trip, styles, and malformed payload fallback.

Notes for reviewer

This is the package prerequisite for the Tightknit app integration. After merge, release-please must publish the next package version before the app PR can replace its local validation link with the npm release.

Tightknit ENG-6388

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