feat: origin-qualified anchors for cross-repo specs

[laulauland]

Summary

When a spec from repo A is installed into repo B (e.g. via skill installation), its drift anchors reference repo A's files. Running drift check in repo B reports false "STALE (file not found)" errors.

This adds an origin field to drift frontmatter:

drift:
  origin: github:fiberplane/drift
  files:
    - src/main.zig@sig:a1b2c3d4e5f67890

When origin is present and doesn't match the current repo's git remote, drift check skips those anchors with SKIP (foreign origin: ...) instead of reporting them as stale. Foreign-origin specs don't cause exit 1.

Stacked on #12.

Changes

• frontmatter.zig — parse origin: from YAML frontmatter and HTML comment blocks. New DriftSpec return type with anchors + origin
• scanner.zig — add origin field to Spec struct
• vcs.zig — add getRepoIdentity() + normalizeGitHubUrl() for matching git remotes to github:owner/repo format
• main.zig — skip foreign-origin specs in lint, show origin in status text/JSON output
• SKILL.md — add origin: github:fiberplane/drift
• docs/ — updated DESIGN.md, CLI.md, DECISIONS.md

Design decisions

• Phase 1 only: no network calls, no fetching from origin repos
• github:owner/repo qualifier scopes identity matching to github.com remotes
• JSON output omits origin when null (backwards-compatible)
• Extensible to other qualifiers and future phases

Full spec: docs/ORIGIN.md

Closes #8

Test plan

• Unit tests for origin parsing (frontmatter + comment blocks)
• Unit tests for GitHub URL normalization
• drift check with foreign-origin specs shows SKIP, exits 0
• drift check in the origin repo checks anchors normally
• drift status displays origin field
• JSON output includes origin only when present