fix(plugins): show load-failure reasons on the page the banner links to

The admin banner counts every plugin load failure, but settings/plugins.html
only rendered failure reasons inside the plugin card macro -- which iterates
discovered plugins. A plugin that failed BECAUSE it has no directory was
therefore counted in the banner and shown nowhere on the page the banner links
to. That dead end is what the operator hit: "1 plugin failed to load" on the
dashboard, nothing wrong on the details page, and no way to clean it up.

Folds the reason into the "Configured but not installed" rows rather than
adding a competing section, and drives that list from the UNION of stored
plugin.* settings and recorded load failures instead of stored settings alone.
A failure with no stored row -- a plugin enabled by a DEFAULTS entry, which is
exactly what plugin.http was -- would otherwise still be counted and still be
invisible.

Rows carry `removable`: only a plugin with a real stored row can be cleaned up,
so a default-declared one shows the reason and an explanation instead of a
Remove button that would delete nothing while the default reasserts it. Rows
with a reason read as an error (red, "Failed to load"); rows that merely have
leftover config stay a warning. A discovered plugin that failed is excluded --
its own card already shows the reason.

Replaces find_orphaned_plugin_names outright rather than adding a second
overlapping helper; it had one caller.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/core/test_plugin_settings_hygiene.py

@@ -8,7 +8,7 @@ CI failure instead of a support question.
 """
 from pathlib import Path
 
-from steward.core.settings import DEFAULTS, find_orphaned_plugin_names
+from steward.core.settings import DEFAULTS, find_orphaned_plugins
 from steward.settings.routes import CAPABILITY_PLUGINS
 
 BUNDLED_PLUGINS_DIR = Path(__file__).resolve().parents[2] / "plugins"
@@ -63,26 +63,73 @@ def test_capability_plugins_all_exist():
 
 
 def test_orphans_are_stored_names_with_no_installed_plugin():
-    assert find_orphaned_plugin_names(
-        ["docker", "traefik", "ancient"], ["docker", "traefik"]
-    ) == ["ancient"]
+    rows = find_orphaned_plugins(["docker", "traefik", "ancient"], ["docker", "traefik"])
+    assert [r["name"] for r in rows] == ["ancient"]
+    assert rows[0]["removable"] is True
+    assert rows[0]["reason"] is None
 
 
 def test_no_orphans_when_everything_is_installed():
-    assert find_orphaned_plugin_names(["docker", "snmp"], ["docker", "snmp"]) == []
+    assert find_orphaned_plugins(["docker", "snmp"], ["docker", "snmp"]) == []
 
 
 def test_installed_plugin_without_stored_settings_is_not_an_orphan():
     # Never configured is not the same as left behind.
-    assert find_orphaned_plugin_names([], ["docker"]) == []
+    assert find_orphaned_plugins([], ["docker"]) == []
 
 
-def test_orphans_are_sorted_and_deduped_of_empties():
-    assert find_orphaned_plugin_names(["zeta", "", "alpha"], []) == ["alpha", "zeta"]
+def test_orphans_are_sorted_and_skip_empty_names():
+    rows = find_orphaned_plugins(["zeta", "", "alpha"], [])
+    assert [r["name"] for r in rows] == ["alpha", "zeta"]
 
 
 def test_accepts_arbitrary_iterables():
     # Callers pass a generator of discovered dirs, not a list.
-    assert find_orphaned_plugin_names(
-        (n for n in ["gone"]), (n for n in ["here"])
-    ) == ["gone"]
+    rows = find_orphaned_plugins((n for n in ["gone"]), (n for n in ["here"]))
+    assert [r["name"] for r in rows] == ["gone"]
+
+
+# ── load failures folded in (issue #2638) ────────────────────────────────────
+
+
+def test_failure_reason_is_attached_to_its_row():
+    rows = find_orphaned_plugins(
+        ["ancient"], [], {"ancient": "Plugin directory not found in: ['/app/plugins']"})
+    assert rows[0]["reason"] == "Plugin directory not found in: ['/app/plugins']"
+
+
+def test_failure_with_no_stored_row_still_gets_a_row():
+    """The banner counts it, so the page must show it — that was the dead end.
+
+    A plugin enabled by a DEFAULTS entry has no stored row, so a stored-only
+    view would leave the banner pointing at a page with nothing on it.
+    """
+    rows = find_orphaned_plugins([], [], {"http": "Plugin directory not found"})
+    assert [r["name"] for r in rows] == ["http"]
+    assert rows[0]["reason"] == "Plugin directory not found"
+
+
+def test_failure_with_no_stored_row_is_not_removable():
+    # Offering Remove would delete nothing while the default reasserts it.
+    rows = find_orphaned_plugins([], [], {"http": "Plugin directory not found"})
+    assert rows[0]["removable"] is False
+
+
+def test_failure_of_an_installed_plugin_is_not_listed_here():
+    # A discovered plugin that failed already shows the reason on its own card.
+    assert find_orphaned_plugins(
+        ["docker"], ["docker"], {"docker": "boom"}) == []
+
+
+def test_stored_orphan_and_failure_are_not_duplicated():
+    rows = find_orphaned_plugins(["gone"], [], {"gone": "Plugin directory not found"})
+    assert len(rows) == 1
+    assert rows[0]["removable"] is True and rows[0]["reason"]
+
+
+def test_every_undiscovered_failure_is_represented():
+    # The invariant that keeps the banner count and this list in agreement.
+    failures = {"a": "x", "b": "y", "docker": "z"}
+    rows = find_orphaned_plugins([], ["docker"], failures)
+    listed = {r["name"] for r in rows}
+    assert listed == {"a", "b"}