feat: import from markdown frontmatter (NEX-003)

[flyingrobots]

Summary

Import nodes and edges from markdown file frontmatter, enabling documentation-as-graph workflows.

Scope

New files

• src/frontmatter.js — Parsing + extraction (~150 LoC)
• test/frontmatter.test.js — Tests (~200 LoC, ~15 tests)

Modified files

• src/import.js — Extract importData(graph, data, opts) from importFile (refactor for reuse)
• src/cli/commands.js — Add importMarkdownCmd
• bin/git-mind.js — Add --from-markdown flag to import case
• src/index.js — Export importFromMarkdown, parseFrontmatter
• CHANGELOG.md

Recognized frontmatter

---
id: doc:architecture
title: "Architecture Guide"
implements: spec:auth
relates-to: [spec:a, spec:b]
depends-on: doc:getting-started
documents: module:auth
---

CLI

git mind import --from-markdown <glob> --dry-run --json

Design decisions

• Auto-generated IDs: doc:<path-without-ext> when id not specified
• Edge type fields in frontmatter map to edge creation
• Simple recursive glob implementation (~30 lines, no new dependencies)
• Refactor: extract importData from importFile for pipeline reuse

Part of NEXUS milestone.