feat: Examark/QTI integration for Canvas LMS export

[Data-Wise]

Summary

Add Canvas LMS export capability to Scholar's teaching commands via Examark (v0.6.6). Two deliverables:

1. New /teaching:canvas converter — transforms existing .qmd exams/quizzes into QTI packages for Canvas import
2. --format canvas flag on existing generation commands (/teaching:exam, /teaching:quiz, /teaching:assignment)

Motivation

Currently, Scholar generates exams/quizzes as .qmd files. To use them in Canvas LMS, instructors must manually re-type questions in the Canvas quiz editor. Examark already converts Markdown to QTI 1.2 (Canvas's import format), and Scholar already has CanvasFormatter and ExamarkFormatter in src/teaching/formatters/ — but they aren't exposed to users or connected to existing .qmd content.

Tracks

Track 1: /teaching:canvas Converter

/teaching:canvas assignments/practice-midterm.qmd
/teaching:canvas assignments/practice-midterm.qmd --validate --emulate

Parses .qmd → detects question types → generates examark .md → runs examark CLI → outputs .qti.zip.

Track 2: --format canvas on Existing Commands

/teaching:exam midterm --format canvas
/teaching:quiz "Week 3" --format canvas

• /teaching:exam has --format in generator but NOT in argument parser — needs fix
• /teaching:quiz --format canvas is documented but untested
• /teaching:assignment has no --format flag

Question Types

All 10 examark types: MC, MA, TF, Short, Numeric, Essay, Matching, FMB, FIB, Upload.

Dependencies

• examark >= 0.6.6 (npm/Homebrew, already installed)
• Node.js >= 18

Spec

Full spec at docs/specs/SPEC-2026-02-26-examark-qti-integration.md

Implementation Order

1. Track 2 (fix --format flags) — ~30 min, existing infrastructure
2. Track 1 (new converter command) — ~2 hours, needs question-type mapping logic