lint: git content-hash drift anchors for code-derived pages (from PR #85)

Roadmap item distilled from PRs #81, #82, and #85 (Mode B code wiki), which could not be rebased past the v2.0.0 refoundation.

Keep the idea, not the mechanism: store a path@sha anchor on pages generated from a repository and let wiki-lint report DRIFTED, MOVED, and UNTRACKED anchors. Implement as a check inside claude_obsidian/lint_engine.py (the 15-skill contract stays), with no watchers, no commit hooks, and no unattended claude -p drain. This completes the "Software repository" scaffold profile described in skills/wiki/references/modes.md and matches the "never fabricate evidence locators" rule in AGENTS.md. The abbreviated and inline code_anchors handling from commit 062a69a in PR #82 is worth preserving verbatim.