feat: include untracked files in commit plans

[rogerchappel]

Summary

• include ordinary non-ignored untracked files in Markdown and JSON plans
• preserve special-character paths with NUL-delimited Git output and deterministic sorting
• report added/untracked metadata and accurate untracked insertion/file counts without mutating the repository
• document every read-only Git query used

Commits

• a3d7c6a feat: include untracked files in commit plans
• 362dec5 docs: document untracked read-only Git queries

Verification

• npm run release:check
• bash scripts/validate.sh
• git diff --check

All 17 tests pass, including untracked-only, mixed tracked/untracked, ignored-file, special-character-path, deterministic-output, and packed-CLI regressions.