feat(operations): cargo cache force purge ("empty all")

purge_cargo_cache gains force: bool. The default age-based purge only
ever removed entries past the configured retention window, which read
as a lie when a user expected "purge" to clear everything -- force=True
sets the cutoff a day in the future so every existing entry is treated
as expired, backing the web UI's explicit "Empty all" action. A
disabled cache stays untouched either way.

src/ci_orchestrator/operations/cache.py

@@ -94,19 +94,33 @@ def cargo_cache_status(config: Config) -> CacheStatus:
 
 
 def purge_cargo_cache(
-    config: Config, *, retention_days: int | None = None, dry_run: bool = False
+    config: Config,
+    *,
+    retention_days: int | None = None,
+    force: bool = False,
+    dry_run: bool = False,
 ) -> PurgeResult:
     """Remove cache entries older than ``retention_days`` (or config's).
 
     Sizes the victims before deleting (a real prune makes their bytes
     unreadable), so ``freed_bytes`` is accurate even for a real purge.
     A disabled cache is left untouched and reports an all-zero result.
+
+    ``force=True`` ignores the retention window entirely and empties the whole
+    cache (every entry, regardless of age) -- this backs the web UI's explicit
+    "Empty all" action, as opposed to the default age-based purge. A disabled
+    cache is still left untouched even under ``force``.
     """
     if not config.cargo_cache.enabled:
         return PurgeResult(removed_entries=0, freed_bytes=0, dry_run=dry_run)
 
-    days = retention_days if retention_days is not None else config.cargo_cache.retention_days
-    cutoff = datetime.now(tz=UTC) - timedelta(days=days)
+    if force:
+        # A cutoff in the future makes every existing entry (mtime <= now)
+        # older than it, so the age-based prune removes the entire cache.
+        cutoff = datetime.now(tz=UTC) + timedelta(days=1)
+    else:
+        days = retention_days if retention_days is not None else config.cargo_cache.retention_days
+        cutoff = datetime.now(tz=UTC) - timedelta(days=days)
     store = CargoCacheStore(config.cargo_cache.root)
 
     doomed = store.prune_older_than(cutoff, what_if=True)

tests/python/test_operations_cache.py

@@ -178,6 +178,41 @@ def test_purge_cargo_cache_nothing_to_purge(tmp_path: Path) -> None:
     assert fresh.exists()
 
 
+def test_purge_cargo_cache_force_removes_everything_regardless_of_age(tmp_path: Path) -> None:
+    root = tmp_path / "cache" / "cargo"
+    fresh = _seed_entry(root, "new", age_days=0)  # brand-new, well within retention
+    mid = _seed_entry(root, "mid", age_days=1)
+    config = _make_config(root, retention_days=30)  # retention alone would spare both
+
+    result = purge_cargo_cache(config, force=True)
+
+    assert result.removed_entries == 2
+    assert not fresh.exists()
+    assert not mid.exists()
+
+
+def test_purge_cargo_cache_force_dry_run_counts_all_but_keeps_files(tmp_path: Path) -> None:
+    root = tmp_path / "cache" / "cargo"
+    fresh = _seed_entry(root, "new", age_days=0, size=2 * 1024 * 1024)
+    config = _make_config(root, retention_days=30)
+
+    result = purge_cargo_cache(config, force=True, dry_run=True)
+
+    assert result == PurgeResult(removed_entries=1, freed_bytes=2 * 1024 * 1024, dry_run=True)
+    assert fresh.exists(), "force dry_run must not delete anything"
+
+
+def test_purge_cargo_cache_force_disabled_still_zero(tmp_path: Path) -> None:
+    root = tmp_path / "cache" / "cargo"
+    old = _seed_entry(root, "old", age_days=1)
+    config = _make_config(root, enabled=False, retention_days=30)
+
+    result = purge_cargo_cache(config, force=True)
+
+    assert result == PurgeResult(removed_entries=0, freed_bytes=0, dry_run=False)
+    assert old.exists()
+
+
 def test_purge_cargo_cache_missing_root_is_not_an_error(tmp_path: Path) -> None:
     root = tmp_path / "cache" / "cargo"
     config = _make_config(root, retention_days=30)