Add stacked markdown review comments

[lmist]

What & why

Adds inline review comments for selected text in the WYSIWYG markdown editor. Users can select arbitrary passages, add multiple anchored comments, review them in the right-side Comments panel, and prepare the collected feedback for agent handoff.

Known issue: in manual testing, clicking Send currently appears to do nothing. This PR leaves that behavior as a follow-up and calls it out explicitly.

How this was verified

Targeted checks passed:

• bun run test:dom src/editor/comments/CommentSelectionButton.dom.test.tsx src/components/DocPanel.dom.test.tsx src/components/DocumentCommentsPanel.dom.test.tsx
• bun test --timeout 30000 --conditions development src/editor/comments/comment-store.test.ts
• bun run typecheck --filter @inkeep/open-knowledge-app
• targeted Biome/Oxlint checks
• git diff --check

Manual UI screenshot:

This screenshot shows selected markdown text highlighted in the editor, stacked comments in the right-side Comments panel, comment editing, and code-block selection highlighting.

Checklist

• Ran bun run check (lint, typecheck, tests) locally
• Added a changeset (bun run changeset) if this changes behavior
• Updated docs if this changes a user-facing surface
• I have read CONTRIBUTING.md and agree to license my contribution under the project's terms (CLA)

[CLAassistant]

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[github-actions]

I could not sync this PR automatically. A maintainer will look into it.

The diff could not be applied on our side. This is a bridge-side issue, not a problem with your PR (which is already based on the latest public base); a maintainer will look into it.

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error: cannot apply binary patch to 'public/open-knowledge/artifacts/stacked-markdown-comments-screenshot.png' without full index line
error: public/open-knowledge/artifacts/stacked-markdown-comments-screenshot.png: patch does not apply
Applied patch to 'public/open-knowledge/packages/app/src/components/DocPanel.dom.test.tsx' cleanly.
Applied patch to 'public/open-knowledge/packages/app/src/components/DocPanel.tsx' cleanly.
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Applied patch to 'public/open-knowledge/packages/app/src/editor/TiptapEditor.tsx' cleanly.
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Applied patch to 'public/open-knowledge/packages/app/src/globals.css' cleanly.
Applied patch to 'public/open-knowledge/packages/app/src/locales/en/messages.json' cleanly.
Applied patch to 'public/open-knowledge/packages/app/src/locales/en/messages.po' cleanly.
Applied patch to 'public/open-knowledge/packages/app/src/locales/pseudo/messages.json' cleanly.
Applied patch to 'public/open-knowledge/packages/app/src/locales/pseudo/messages.po' cleanly.

[miles-kt-inkeep]

Hey @lmist, thanks for the contribution. We have some internal work ongoing for this feature, but we will take your work into account as well.