Accept git+ sources for the sync/prune env-definition positional

[OwenPriceSkelly]

Summary

rootstock sync and rootstock prune take an optional positional source of env definitions; until now it had to be a local staging directory. This adds a second form — a pip-style git spec:

rootstock sync 'git+https://github.com/Garden-AI/rootstock.git@main#subdirectory=sample_model_configurations/nvidia_configs' --cluster delta

REF may be a branch, tag, or full commit SHA (omitted = the remote default branch); subdirectory defaults to the repo root. This lets a cluster converge straight from the catalog repo, skipping the scp/staging round-trip.

How

• New rootstock/gitsource.py: spec parsing plus a shallow fetch (init + fetch --depth 1 + detached FETCH_HEAD checkout — one code path that covers branch/tag/SHA refs, unlike clone --branch) into a temp dir removed at process exit. That lifetime is sufficient: the planner hashes the staged files and the build phase copies them into the install root, so nothing durable lives in the checkout.
• Shared resolve_source_arg in commands/common.py dispatches git specs vs. local dirs; the sync and prune adapters both use it, preserving the existing contract (unusable source → exit 2).
• Everything downstream of resolution is untouched — the checkout flows through the same source_dir path the planner already had.
• Clean usage errors for: missing git binary, malformed specs, unknown fragment options, refs that don't exist (git stderr attached), and subdirectory values that are missing or escape the repo.

Testing

• tests/test_gitsource.py: parse cases (ssh userinfo vs. ref @, branch names with /, malformed specs) and real fetches against a local repo over file:// (default branch, tag, SHA, subdirectory, error paths).
• tests/cli/test_sync.py: adapter wiring (git spec materialized before planning; fetch failure → exit 2).
• Full tests/cli + tests/commands suites pass (304 tests); ruff check/format and ty clean.
• Manual end-to-end: dry-run sync from a scratch git+file://…#subdirectory=environments repo produced the expected build/download plan.

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