Improve ACPX adapter configuration (#5290)

## Thinking Path

> - Paperclip orchestrates AI agents across several adapter
implementations.
> - ACPX is a local adapter path that can proxy Claude and Codex-style
execution.
> - Its configuration needed stronger schema defaults, provider-aware
model handling, and better UI support.
> - Plugin authors also need clear docs for managed resources.
> - This pull request improves ACPX adapter configuration and documents
plugin-managed resources.
> - The benefit is a more predictable adapter setup path without
changing unrelated control-plane behavior.

## What Changed

- Improved ACPX config schema, execution config handling, UI build
config, and route coverage.
- Added ACPX model filtering support and tests.
- Updated the agent config form and storybook coverage for ACPX
model/provider behavior.
- Expanded plugin authoring documentation for managed resources.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm exec vitest run server/src/__tests__/acpx-local-execute.test.ts
server/src/__tests__/adapter-routes.test.ts
ui/src/lib/acpx-model-filter.test.ts`

## Risks

- Low-to-medium risk: adapter configuration behavior changes can affect
ACPX users, but the change is isolated to ACPX/plugin-doc surfaces and
covered by targeted adapter tests.

## Model Used

- OpenAI GPT-5 Codex via Paperclip `codex_local` adapter, with
shell/git/GitHub CLI tool use.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

server/src/__tests__/adapter-models.test.ts

@@ -3,7 +3,7 @@ import { models as codexFallbackModels } from "@paperclipai/adapter-codex-local"
 import { models as cursorFallbackModels } from "@paperclipai/adapter-cursor-local";
 import { models as opencodeFallbackModels } from "@paperclipai/adapter-opencode-local";
 import { resetOpenCodeModelsCacheForTests } from "@paperclipai/adapter-opencode-local/server";
-import { listAdapterModels, refreshAdapterModels } from "../adapters/index.js";
+import { listAdapterModels, listServerAdapters, refreshAdapterModels } from "../adapters/index.js";
 import { resetCodexModelsCacheForTests } from "../adapters/codex-models.js";
 import { resetCursorModelsCacheForTests, setCursorModelsRunnerForTests } from "../adapters/cursor-models.js";
 
@@ -30,6 +30,13 @@ describe("adapter model listing", () => {
     expect(models).toEqual([]);
   });
 
+  it("uses provider-prefixed ACPX fallback model labels", () => {
+    const adapter = listServerAdapters().find((candidate) => candidate.type === "acpx_local");
+
+    expect(adapter?.models?.some((model) => model.label.startsWith("Claude: "))).toBe(true);
+    expect(adapter?.models?.some((model) => model.label.startsWith("Codex: "))).toBe(true);
+  });
+
   it("returns codex fallback models when no OpenAI key is available", async () => {
     const fetchSpy = vi.spyOn(globalThis, "fetch");
     const models = await listAdapterModels("codex_local");