fix(plugins): keep the salt working where hardlinks are not supported

Self-review of the atomic-publish fix. Some network mounts and container volumes
reject os.link, and the outer handler swallowed that into "no salt", which meant
repo_hash and session_hash were dropped on every run for that whole cohort. The
race being closed is narrow; losing the hashes for an entire filesystem is not a
fair trade.

Falls back to claiming the name with O_CREAT|O_EXCL and writing, which is what
this did before. The empty-file window reopens there, but it is benign now: a
reader landing in it gets "" and omits the hash for that process rather than
caching a guessable path digest, which was the actual defect.

266 passed, 8 skipped.

Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb

integrations/claude-code-plugin/core/telemetry.py

@@ -147,6 +147,19 @@ def _install_salt() -> str:
             os.link(temporary, path)
         except FileExistsError:
             pass
+        except OSError:
+            # No hardlinks here (some network mounts, some container volumes).
+            # Claim the name directly instead. That reopens the empty-file
+            # window, but the window is now benign: a reader that lands in it
+            # gets "" and omits the hash for that process rather than caching a
+            # guessable one. Losing the hashes on every run of an entire
+            # filesystem is the worse failure.
+            try:
+                fallback = os.open(path, os.O_CREAT | os.O_EXCL | os.O_WRONLY, 0o600)
+                with os.fdopen(fallback, "w", encoding="utf-8") as stream:
+                    stream.write(temporary.read_text(encoding="utf-8"))
+            except OSError:
+                pass
     except OSError:
         pass
     finally:

integrations/claude-code-plugin/tests/test_telemetry.py

@@ -352,6 +352,26 @@ def test_a_half_written_salt_is_never_visible_to_another_process(isolated_env, m
     assert salt_path.read_text(encoding="utf-8").strip() == salt
 
 
+def test_a_filesystem_without_hardlinks_still_gets_a_salt(isolated_env, monkeypatch):
+    """Publishing by link must not become a silent loss of the hashes.
+
+    Some network mounts and container volumes reject os.link. Returning ""
+    there would drop repo_hash and session_hash on every run for that whole
+    cohort, which is a bigger loss than the narrow race the link closes.
+    """
+    telemetry._salt_cache = ""
+    monkeypatch.setattr(
+        telemetry.os, "link", lambda src, dst: (_ for _ in ()).throw(OSError(38, "not implemented"))
+    )
+
+    salt = telemetry._install_salt()
+
+    assert len(salt) == 32, "no salt on a filesystem without hardlinks"
+    assert telemetry._salt_path().read_text(encoding="utf-8").strip() == salt
+    assert telemetry._scoped_digest("git@github.com:acme/x.git") != ""
+    assert not list(telemetry._salt_path().parent.glob("telemetry-salt.*.tmp"))
+
+
 def test_a_concurrent_writer_does_not_clobber_the_published_salt(isolated_env):
     """Second process to finish must adopt the first one's salt, not replace it.