perf(cli): skip plugin load on agents list --json (#71739) (#71746)

Reporter measured `agents list --json` at ~7-9s on a fast host (~11s in
container) on 2026.4.23, while peer `--json` commands like
`channels list`, `cron list --all`, and `sessions ... --all-agents`
stay sub-second. Their cold-call dashboard endpoint dropped from 27s to
~2s after a local dist patch — they could even retire the 5-min cache
TTL workaround they had shipped to dodge it.

Root cause: `agents list` inherits `loadPlugins: 'always'` from the
parent `agents` policy in command-catalog, then `agentsListCommand`
calls `buildProviderStatusIndex(cfg)` unconditionally — both paths
trigger the bundled-extension import waterfall (~60+ extension index.js
modules).

`channels list` already uses `loadPlugins: 'never'` and proves the
shape is right; this PR matches that shape with the safer `text-only`
variant so human invocations are unchanged.

Two-line fix per reporter:

1. `src/cli/command-catalog.ts` — opt agents list into `text-only`,
   the same plugin-preload policy bucket that already exists. Plugin
   preload runs for human text output, skips for `--json`.

2. `src/commands/agents.commands.list.ts` — skip
   `buildProviderStatusIndex` (and the per-summary provider
   enrichment loop) when `opts.json`. Provider info is only rendered
   in human text output via `formatSummary`, so dropping it from JSON
   has no observable effect on existing callers that consume `id`,
   `name`, `model`, `bindings`, `isDefault`, `identity*`, `workspace`,
   or `agentDir`. `routes` is config-derived and continues to be set
   in both modes.

Tests:
- new assertion in command-startup-policy.test.ts: `agents list` with
  jsonOutputMode:true now resolves to `loadPlugins: false` (was
  effectively `true` via the parent `agents` 'always' policy).
- existing assertion that human (jsonOutputMode:false) still triggers
  plugin load is preserved verbatim.

6/6 tests pass. Lint clean.

Out of scope:
- `--bindings` flag opt-in for restoring providers in JSON output:
  worth adding later if any consumer needs it; reporter said dashboard
  consumers don't.
- Broader plugin-discovery cache work (#67040, #71690) which addresses
  the same family of cold-start cost.

src/cli/command-catalog.ts

@@ -73,6 +73,12 @@ export const cliCommandCatalog: readonly CliCommandCatalogEntry[] = [
   },
   {
     commandPath: ["agents", "list"],
+    // JSON callers (dashboards, monitoring scripts, IDE plugins) poll this
+    // command and don't need the plugin-derived `providers` enrichment that
+    // is only used in human text output. text-only skips the bundled-plugin
+    // import waterfall in `--json` mode, mirroring what `channels list`
+    // already does. Human (non-JSON) invocations still load plugins. (#71739)
+    policy: { loadPlugins: "text-only" },
     route: { id: "agents-list" },
   },
   {

src/cli/command-startup-policy.test.ts

@@ -80,6 +80,15 @@ describe("command-startup-policy", () => {
         jsonOutputMode: false,
       }),
     ).toBe(true);
+    // text-only opts agents list out of plugin preload in --json mode so
+    // dashboards/scripts that poll this command don't pay the bundled-plugin
+    // import waterfall when they only consume config-derived fields. (#71739)
+    expect(
+      shouldLoadPluginsForCommandPath({
+        commandPath: ["agents", "list"],
+        jsonOutputMode: true,
+      }),
+    ).toBe(false);
   });
 
   it("matches banner suppression policy", () => {

src/commands/agents.commands.list.ts

@@ -99,7 +99,14 @@ export async function agentsListCommand(
     }
   }
 
-  const providerStatus = await buildProviderStatusIndex(cfg);
+  // `buildProviderStatusIndex` triggers on-demand plugin loads and is only
+  // used for human text output (`summary.providers` is rendered in the text
+  // formatter). JSON callers (dashboards, monitors, IDE plugins) poll this
+  // command and don't need provider enrichment, so skip the plugin load when
+  // emitting JSON — combined with `loadPlugins: "text-only"` in the catalog
+  // entry, this drops `agents list --json` cold time from ~9s to sub-second.
+  // (#71739)
+  const providerStatus = opts.json ? null : await buildProviderStatusIndex(cfg);
 
   for (const summary of summaries) {
     const bindings = bindingMap.get(summary.id) ?? [];
@@ -110,14 +117,16 @@ export async function agentsListCommand(
       summary.routes = ["default (no explicit rules)"];
     }
 
-    const providerLines = listProvidersForAgent({
-      summaryIsDefault: summary.isDefault,
-      cfg,
-      bindings,
-      providerStatus,
-    });
-    if (providerLines.length > 0) {
-      summary.providers = providerLines;
+    if (providerStatus) {
+      const providerLines = listProvidersForAgent({
+        summaryIsDefault: summary.isDefault,
+        cfg,
+        bindings,
+        providerStatus,
+      });
+      if (providerLines.length > 0) {
+        summary.providers = providerLines;
+      }
     }
   }