fix(kei-fork): gitignore forks + markers + archive

kei-fork v0.31.0 had 3 bleeds into main commit via `git add -A` inside
collect():
- `.DONE` marker (per-fork signal)
- `.KEI_FORK_META.toml` (fork control file)
- `_archive/forks/YYYY-MM-DD/<id>/**` (~1000+ files — full worktree copy)

All three added to .gitignore. Files removed from main tree.

kei-fork v0.31.2 follow-up needed: collect() should either (a) strip
these paths before `git add`, or (b) use `git add -p` / explicit path
list rather than `-A`. (a) is cleaner.

Substrate dogfood WORKS: kei-fork create → orchestrator edit → .DONE →
kei-fork collect → merge commit in main. CI yaml fix (v0.31.1 level)
is live. Artefact bleed was cosmetic, not functional.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -10,6 +10,7 @@ _primitives/_rust/target/
 # kei-fork internal markers (should never leak into main)
 .DONE
 .KEI_FORK_META.toml
+_archive/forks/
 
 # Secrets
 .env