refactor(infra): worktree-safe shallow snapshot fetch, drop $HOME/param-decomp

[claude-spd1]

What

generate_git_snapshot_setup in param_decomp_lab/infra/slurm.py (harvest /
autointerp / investigate SLURM jobs) cloned "$HOME/param-decomp" and did a full
git fetch <ref>:<ref>. That $HOME/param-decomp hardcode is an unblessed hack from
commit 24190c261 (torch era), whose motivation — generating an sbatch from inside a
SLURM job (async slow-eval via sink.on_save), where REPO_ROOT points at a node-local
/tmp/.../workspace-* — is dead on feature/jax.

Reconsidered in light of #960

#960 already returned the pd-lm trainer launch to the same job-side setup pattern
these jobs never left, and resolved the fetch source with
git rev-parse --path-format=absolute --git-common-dir (the main checkout's .git on
shared FS, worktree-safe). So rather than land a second copy of that helper, this PR
promotes it to a single shared home and points both launchers at it:

• infra/git.py: new snapshot_source_repo() — the one definition (it already hosts
  the other REPO_ROOT-based git utilities both launchers import).
• infra/slurm.py: generate_git_snapshot_setup now does git init +
  git fetch --quiet --depth 1 "file://<common-dir>" <ref> + git checkout --quiet FETCH_HEAD, replacing the $HOME/param-decomp clone + tag/head fetch. file:// is
  required — git silently ignores --depth on the bare-path local transport. .env
  (untracked secrets) is copied from the durable main checkout (the common git dir's
  parent worktree), not the possibly-ephemeral submitting worktree.
• experiments/lm/launch.py: drops its private _snapshot_source_repo (added in
  refactor(pd-lm): job-side node workspaces, drop the submit-time shared-FS build #960), imports the shared one — net −16 lines there.

Result: one implementation of "where does a job fetch its snapshot from", used by both
the trainer launch and the shared SLURM fragment, instead of two.

Testing

• New param_decomp_lab/tests/test_slurm.py asserts the fragment shape (shallow file://
  fetch from the common dir, FETCH_HEAD checkout, durable .env, no git clone, no
  $HOME/param-decomp).
• pytest test_slurm.py test_launch.py → 9 passed; ruff check/format clean;
  basedpyright 0 errors on all four touched files.
• Runtime sanity: slurm.snapshot_source_repo is git.snapshot_source_repo (single
  object), and rendering the fragment from inside a linked worktree resolves the common
  dir to the main checkout /mnt/home/pd-user/param-decomp/.git (worktree-safety).

Follow-up from #960. Board task harvest-fragment-drop-home-checkout.

Crew-Address: task/harvest-fragment-drop-home-checkout