fix(core): reduce snapshot Git scans - #43774

AI code review — automated review for reference; please use your judgment.

• packages/core/src/git.ts:782-816 — parseStatus is a pure, intricate parser (rename pairs consume the following NUL-separated origPath via index++, fixed-width field math in statusPath) yet ships with zero unit tests. Please add direct cases: format-2 renames, filenames containing spaces (relies on skipping exactly N separators), unmerged u rows (10-field prefix), submodule rows, truncated/malformed rows, and empty output.
• packages/core/src/git.ts:798-800 — excluding staged-only changes (Y === '.') intentionally mirrors the old diff-files semantics; that invariant is load-bearing but easy to break — add a comment noting "worktree-side deltas only" next to the changed computation.
• packages/core/src/git.ts:426-431 — -c diff.ignoreSubmodules=untracked changes behavior beyond saving a process: untracked content inside submodules will no longer appear as snapshot candidates. If intentional, please state it in the PR description; if not, drop the config override.
• packages/core/src/git.ts:801-803 — on truncated rows submodule[1] !== '.' evaluates against undefined, so changed fails open and the path is still included — safe direction (extra hashing, never missed changes), but a field-count sanity check before slicing would make silent corruption visible.
• packages/core/src/git.ts:463 — nit: Array.from(allowed) materializes a second intermediate array; building the complement directly while constructing allowed would avoid it. Trivial.

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