OI-T1: Callout node (first-class, round-trip)

[coderhd]

Parent

#27 — H0 — Obsidian Importer

What to build

A first-class callout node so Obsidian > [!note] syntax is preserved, not flattened. The Tiptap Callout extension joins the shared schema (extracted by MI-T1) and the round-trip engine serializes/parses the Obsidian-compatible > [!type] title form, with styled rendering in the editor.

Acceptance criteria

• Callout Tiptap extension added to lib/editor-extensions.ts (the shared schema); the live editor, paste path, and round-trip engine all share it.
• Serialization emits > [!type] title (Obsidian-compatible, e.g. note/warning/info); parsing recognizes callouts on import and in pasted markdown.
• Callouts render with type-based styling in the editor.
• Round-trip stability: callout doc → markdown → callout doc (and the reverse) is lossless.
• Focused unit tests pass; npm run lint and npm run build green.

[coderhd]

Implementation complete. PR: #75

All 6 plan tasks done — 17 new callout tests, 301 total green, lint/tsc/build clean. Per-task + final whole-branch reviews passed; two plan-level input-rule defects were found during execution and fixed (Enter-trigger redesign validated by probe; export collapsed-hide CSS added).

[coderhd]

Merged in #75 (commit de5483e). Closing — callout node is live. #74's blocked_by edge auto-resolves.