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>

steward/core/settings.py

@@ -18,7 +18,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
-from collections.abc import Iterable
+from collections.abc import Iterable, Mapping
 from datetime import datetime, timezone
 from typing import Any
 
@@ -267,20 +267,42 @@ async def get_stored_plugin_names(session: AsyncSession) -> list[str]:
     )
 
 
-def find_orphaned_plugin_names(
+def find_orphaned_plugins(
     stored_names: Iterable[str],
     installed_names: Iterable[str],
-) -> list[str]:
-    """Stored plugin settings whose plugin is not currently installed.
+    failures: Mapping[str, str] | None = None,
+) -> list[dict[str, Any]]:
+    """Plugins that are configured or failed to load, but are not installed.
 
     Pure so it can be tested without a DB or a filesystem. "Not installed" is
     intentionally not treated as "safe to delete" — an external plugin under
     /data/plugins can be missing merely because the volume isn't mounted or an
     install failed, so the caller surfaces these for an explicit operator
     decision instead of removing them automatically.
+
+    Driven by the UNION of stored settings and load failures, not by stored
+    settings alone. `load_plugins` records a failure for any enabled plugin
+    whose directory it cannot find, and the admin banner counts those — so a
+    failure with no stored row (a plugin enabled by a DEFAULTS entry) would
+    otherwise be counted in the banner and shown nowhere, which is exactly the
+    dead end this section exists to close.
+
+    Each row carries `removable`: only a plugin with a real stored row can
+    actually be cleaned up. Offering Remove for a default-declared plugin would
+    be a lie — deleting nothing, while the default reasserts it on next load.
     """
     installed = set(installed_names)
-    return sorted(name for name in stored_names if name and name not in installed)
+    failures = dict(failures or {})
+    stored = {name for name in stored_names if name}
+    candidates = (stored | set(failures)) - installed
+    return [
+        {
+            "name": name,
+            "reason": failures.get(name),
+            "removable": name in stored,
+        }
+        for name in sorted(candidates)
+    ]
 
 
 async def get_all_settings(session: AsyncSession) -> dict[str, Any]: