feat: explicit mount control with MountSpec pairs

[TimBeyer]

Summary

Closes #13 — the home directory is not mounted by default. Users opt
in explicitly, and now have full control over what gets mounted where.

• Replace string[] mounts with explicit MountSpec objects
  ({ location, mountPoint, writable? }) — no magic path derivation,
  users specify exactly where things land in the guest
• Wizard configure step asks Y/n for home directory mount (default: No);
  answering yes sets { location: "~", mountPoint: "/mnt/host" }
• Headless config uses the same object syntax
• Lima YAML generation rewritten to use the yaml npm package — builds
  a data structure instead of string templates, eliminating indentation bugs
• Tests parse YAML output structurally instead of string-matching

Before (string-based, magic derivation)

"mounts": ["~", "~/.ssh"]

After (explicit pairs)

"mounts": [
  { "location": "~", "mountPoint": "/mnt/host" },
  { "location": "/opt/data", "mountPoint": "/mnt/data", "writable": true }
]

Test plan

• bun test — all 249 tests pass
• bun run lint && bun run format:check — clean
• Manual: wizard → decline home mount → no extra mounts in YAML
• Manual: wizard → accept home mount → ~ at /mnt/host in YAML
• Headless: "mounts": [{ "location": "~", "mountPoint": "/mnt/host" }] → mount appears
• Headless: no mounts field → no extra mounts

Closes #13

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