refactor(slurm): fetch snapshots from the git common dir, drop $HOME/param-decomp

[claude-spd1]

Follow-up from #960. generate_git_snapshot_setup in param_decomp_lab/infra/slurm.py (harvest / autointerp / investigate jobs) still cloned a hard-coded $HOME/param-decomp at shell-time — a hack from 24190c2 whose motivation (torch-era async slow-eval sbatching from inside a SLURM job, where REPO_ROOT resolved to the submitter's node-local /tmp workspace) is dead on feature/jax.

What changed

Aligned it with pd-lm's node-workspace pattern (experiments/lm/launch.py::_snapshot_source_repo / _node_workspace_setup):

• Resolve the submitting checkout's common git dir at submit via git rev-parse --path-format=absolute --git-common-dir (new _snapshot_source_repo helper). Worktree-safe: a linked worktree shares the main checkout's .git, so a snapshot survives the worktree being deleted before a requeue re-fetches.
• In-job: git init + shallow git fetch --depth 1 "file://<common dir>" <ref> + git checkout FETCH_HEAD, replacing the git clone/git fetch refs pair. file:// (not the bare path) so git honours --depth — it silently ignores it on the bare-path local transport.
• .env (secrets, not in git) now copied from the durable main checkout that owns the common dir, not the possibly-ephemeral submitting worktree.

Shared FS throughout, so neither submit nor requeue depends on GitHub reachability (the origin push in create_git_snapshot stays a best-effort provenance backup).

Only caller is _workspace_setup (via SlurmConfig.snapshot_ref); signature unchanged, so harvest/autointerp/investigate need no edits.

Tests

Adds param_decomp_lab/tests/test_slurm.py: asserts the fragment fetches from the common dir via file:// with --depth 1, checks out FETCH_HEAD, copies the durable .env, has no git clone / $HOME/param-decomp, and that _snapshot_source_repo resolves an absolute existing path. ruff check + ruff format --check clean.

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

[claude-spd1]

Duplicate of #963 (two parallel crew bodies opened equivalent PRs ~30s apart; identical code, minor docstring/test-text differences only). Consolidating on #963. Crew-Address: task/harvest-fragment-drop-home-checkout