feat(adapters): GitShardAdapter write=commit + current_rev drift (WP-0012 T2)

Writable mode: write(key, body) stages and commits the file (skipping a no-op so
no empty commit is created), returning the page at the new commit sha. The
writable profile declares WRITE + VERSION with PER_PAGE granularity. current_rev
is the per-path commit sha, so a write — or an external commit to the same path —
moves it, driving apply-under-drift. Passes the conformance positive-write probe.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_git_adapter_write.py

@@ -0,0 +1,89 @@
+"""Tests for GitShardAdapter write=commit + current_rev drift (SHARD-WP-0012 T2)."""
+
+import os
+import subprocess
+
+from shard_wiki.adapters import GitShardAdapter, run_conformance
+from shard_wiki.model import Verb
+
+_ENV = {
+    "GIT_AUTHOR_NAME": "t", "GIT_AUTHOR_EMAIL": "t@t",
+    "GIT_COMMITTER_NAME": "t", "GIT_COMMITTER_EMAIL": "t@t",
+    "PATH": os.environ.get("PATH", ""),
+}
+
+
+def _git(repo, *args, capture=False):
+    return subprocess.run(
+        ["git", "-C", str(repo), *args], check=True, capture_output=True, text=True, env=_ENV
+    ).stdout.strip()
+
+
+def _repo(tmp_path, files):
+    repo = tmp_path / "repo"
+    repo.mkdir()
+    _git(repo, "init", "--quiet")
+    for rel, text in files.items():
+        (repo / rel).write_text(text, encoding="utf-8")
+        _git(repo, "add", rel)
+    _git(repo, "commit", "-m", "seed")
+    return repo
+
+
+def test_writable_profile_declares_write_and_version(tmp_path):
+    profile = GitShardAdapter("git", _repo(tmp_path, {"Home.md": "h"}), writable=True).profile()
+    assert profile.supports(Verb.WRITE)
+    assert profile.supports(Verb.VERSION)
+    profile.validate()  # PER_PAGE + WRITE is a consistent combination
+
+
+def test_write_creates_a_commit(tmp_path):
+    repo = _repo(tmp_path, {"Home.md": "old"})
+    adapter = GitShardAdapter("git", repo, writable=True)
+    before = _git(repo, "rev-list", "--count", "HEAD")
+    page = adapter.write("Home", "new body")
+    after = _git(repo, "rev-list", "--count", "HEAD")
+    assert int(after) == int(before) + 1  # one new commit
+    assert page.body == "new body"
+    assert page.envelope.source_rev == _git(repo, "rev-parse", "HEAD")  # page is at the new sha
+
+
+def test_write_advances_current_rev(tmp_path):
+    repo = _repo(tmp_path, {"Home.md": "old"})
+    adapter = GitShardAdapter("git", repo, writable=True)
+    rev_before = adapter.current_rev("Home")
+    adapter.write("Home", "changed")
+    assert adapter.current_rev("Home") != rev_before  # sha moved → drift detectable
+
+
+def test_write_new_key_tracks_it(tmp_path):
+    repo = _repo(tmp_path, {"Home.md": "h"})
+    adapter = GitShardAdapter("git", repo, writable=True)
+    adapter.write("docs/New", "fresh page")
+    assert "docs/New" in set(adapter.keys())
+    assert adapter.read("docs/New").body == "fresh page"
+
+
+def test_noop_write_creates_no_empty_commit(tmp_path):
+    repo = _repo(tmp_path, {"Home.md": "same"})
+    adapter = GitShardAdapter("git", repo, writable=True)
+    before = _git(repo, "rev-list", "--count", "HEAD")
+    adapter.write("Home", "same")  # identical body → nothing to commit
+    assert _git(repo, "rev-list", "--count", "HEAD") == before
+
+
+def test_current_rev_reflects_external_commit(tmp_path):
+    repo = _repo(tmp_path, {"Home.md": "h"})
+    adapter = GitShardAdapter("git", repo, writable=True)
+    rev = adapter.current_rev("Home")
+    # An out-of-band commit to the same path (another writer) moves the per-path sha.
+    (repo / "Home.md").write_text("externally edited", encoding="utf-8")
+    _git(repo, "add", "Home.md")
+    _git(repo, "commit", "-m", "external")
+    assert adapter.current_rev("Home") != rev
+
+
+def test_conformance_positive_write_probe_passes(tmp_path):
+    adapter = GitShardAdapter("git", _repo(tmp_path, {"Home.md": "body"}), writable=True)
+    report = run_conformance(adapter)
+    assert report.ok, report.diff()