Add a grut status command for scriptable repo state

[jongio]

What

Adds a grut status subcommand that prints a summary of the current repository state, with a --json flag for scripting.

Text output shows the branch, upstream tracking (ahead/behind), and the staged, unstaged, untracked, and conflicted files. JSON output is a stable document with the same information plus counts, so tools and scripts can parse it without scraping the human format.

$ grut status
On branch main
Tracking origin/main [ahead 1]
Staged (1):
  M cmd/status.go
Untracked (1):
  ? notes.txt

$ grut status --json
{
  "branch": "main",
  "upstream": "origin/main",
  "ahead": 1,
  "behind": 0,
  "detached": false,
  "clean": false,
  "staged": [{ "path": "cmd/status.go", "status": "M" }],
  "unstaged": [],
  "untracked": ["notes.txt"],
  "conflicted": [],
  "counts": { "staged": 1, "unstaged": 0, "untracked": 0, "conflicted": 0 }
}

How

• New StatusWithBranch method on the git client returns the branch header and file entries from a single porcelain v2 invocation, so the command does not run git status twice.
• buildStatusReport classifies entries into staged, unstaged, untracked, and conflicted buckets. A file that is changed in both the index and the worktree appears in both staged and unstaged.
• Detached HEAD is detected from the porcelain header and reported as such.

Testing

• Unit tests for classification, clean and detached states, text output sections, and JSON round-trip.
• Integration test that runs the command against a real temporary repo (guarded on git being available).
• go build ./..., go vet ./..., and gofumpt all clean.

Closes #314