feat(editor): slash command menu (PRSM-47)

[nchopra]

Summary

Notion-style `/` command menu for the TipTap note editor. Built on `@tiptap/suggestion` + a custom React popover. Renderer-only — no schema changes, no markdown round-trip impact.

Items (10): Heading 1/2/3, Bullet list, Numbered list, Check list, Quote, Code block, Divider, Table. No skill / artifact items — those have their own surfaces.

Trigger: `/` at the start of an empty paragraph (optionally after a single leading space). Mid-sentence `/` does not fire. Disabled inside `tableCell` / `tableHeader` / `artifact` wrappers.

Notable design choices

• Renderer-only. Mounted in `buildRendererExtensions`, NOT in `buildEditorExtensions` (the headless schema). Markdown export/import and Yjs collab schema are untouched.
• `set` not `toggle`.** Headings/quote/code-block items use `setHeading` / `setBlockquote` / `setCodeBlock`. Lists keep `toggle*` (TipTap 3.x list extensions don't expose `set*` variants) but are guarded by `editor.isActive(...)` — picking the type you're already in is a no-op instead of demoting.
• Viewport clamp + flip-up. Popover flips above the caret when there's no room below; left edge clamps inside the viewport.

After Opus adversarial review

Opus surfaced 3 required + 3 nice-to-have items; all 6 addressed in commit `bd5283b`:

• viewport clamp + flip
• `allow` walks ancestors to exclude unwanted parents
• set-not-toggle (with isActive guard for lists)
• tighter `Suggestion<SlashMenuItem, SlashMenuItem>` generic
• `role="listbox"` on the `
  `
  • dropped dead Escape branch in `onKeyDown`

  Test plan

  Automated:

  • `pnpm -F desktop type:check` — clean
  • `pnpm -F desktop test --run tests/editor/` — 25 tests pass (8 files; 3 new in `slash-menu-items.test.ts`)

  Manual smoke (please verify in dev):

  • Type `/` at start of an empty paragraph at the top, middle, and bottom of the viewport — popover appears; flips up near the bottom.
  • Mid-sentence `/` does NOT trigger.
  • Inside a table cell paragraph: `/` does NOT trigger.
  • Inside an artifact block: `/` does NOT trigger.
  • Each item produces the expected node tree; trigger text is replaced cleanly.
  • Inside an existing bullet list, picking "Bullet list" is a no-op (does NOT demote to paragraph).
  • ArrowUp/ArrowDown navigates; Enter applies; Esc dismisses.
  • Click selects.