feat(RMGR-WP-0001): complete T05 end-to-end vertical slice

Implement observe/reconcile/update-task-status CLI path: workplan parse,
JSON projection index, git-backed task status writeback with correlation
events, and E2E pytest plus evidence artifacts. Finish foundation workplan.

src/repo_manager/gitops.py

@@ -0,0 +1,66 @@
+"""Git helpers via subprocess (ADR-001)."""
+
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+
+class GitError(RuntimeError):
+    pass
+
+
+def _run(repo: Path, *args: str, check: bool = True) -> subprocess.CompletedProcess[str]:
+    proc = subprocess.run(
+        ["git", *args],
+        cwd=repo,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if check and proc.returncode != 0:
+        raise GitError(proc.stderr.strip() or proc.stdout.strip() or f"git {' '.join(args)} failed")
+    return proc
+
+
+def head_sha(repo: Path) -> str | None:
+    proc = _run(repo, "rev-parse", "HEAD", check=False)
+    if proc.returncode != 0:
+        return None
+    return proc.stdout.strip() or None
+
+
+def is_git_repo(repo: Path) -> bool:
+    return (repo / ".git").exists() or _run(repo, "rev-parse", "--git-dir", check=False).returncode == 0
+
+
+def commit_paths(
+    repo: Path,
+    paths: list[str],
+    message: str,
+    *,
+    author_name: str = "repo-manager",
+    author_email: str = "repo-manager@local",
+) -> str:
+    """Stage paths and commit. Returns new HEAD sha."""
+    env_author = [
+        "-c",
+        f"user.name={author_name}",
+        "-c",
+        f"user.email={author_email}",
+    ]
+    for path in paths:
+        _run(repo, "add", "--", path)
+    proc = subprocess.run(
+        ["git", *env_author, "commit", "-m", message],
+        cwd=repo,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if proc.returncode != 0:
+        raise GitError(proc.stderr.strip() or proc.stdout.strip() or "git commit failed")
+    sha = head_sha(repo)
+    if not sha:
+        raise GitError("commit succeeded but HEAD missing")
+    return sha