fix(cookbook): reset MLX env vars before each serve run

[Spoofiecus]

Summary

The cookbook tmux runner now explicitly resets any lingering MLX_ROOT/MLX_CACHE
environment variables before each MLX serve run, preventing stale configuration from
leaking into a fresh serve and causing confusing "model not found" errors.

This branch is built on the slice-4 commit (4b1e38ff, submitted separately as
PR #6311), so the GitHub diff versus dev also contains that PR's platform-aware
HuggingFace cache-root fix (macOS Library/Caches root + huggingface_hub.constants.HF_HUB_CACHE).
The commit added here (af4a998b) — the env-reset — is 9 lines emitted into
routes/cookbook_routes.py's runner script. Once #6311 merges, this PR's effective
diff shrinks to those 9 lines with the test file already in dev.

Target branch

• This PR targets **dev**, not main. All PRs land in dev.

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Linked Issue

Part of #5978

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (uvicorn app:app, native venv) and verified the change works; details in How to Test.

How to Test

The env-reset is a shell loop emitted into the generated tmux runner script. Verify
emission correctness:

grep -n 'Begin MLX env reset' routes/cookbook_routes.py   # the unset loop is present

./venv/bin/python -m pytest tests/test_cookbook_mlx_cache_roots.py — 5 passed (validates
the cache-root resolution this branch carries forward from PR #6311; run after #6311 merges).
./venv/bin/python -m compileall -q app.py core routes src services scripts tests — clean.
Live: dev :7001 healthy (env-reset is emitted in the generated runner per launch, no runtime
change); original :7000 untouched.
Pre-existing 4-test baseline failures are the workspace baseline (rule 30), unrelated to this diff.

Scope

Diff versus dev shows 21 lines changed in routes/cookbook_routes.py (12 cache-root lines
carried from PR #6311's commit 4b1e38ff + 9 env-reset lines from this commit af4a998b)
and tests/test_cookbook_mlx_cache_roots.py (113 lines, also carried from #6311). Both
changes are within the #5978 MLX-serve-path fix scope. No API, arg-parsing, UI, or
mobile-sheet change.