Generalize sandbox provider core for plugin-only providers (#4449)

## Thinking Path

> - Paperclip is a control plane, so optional execution providers should
sit at the plugin edge instead of hardcoding provider-specific behavior
into core shared/server/ui layers.
> - Sandbox environments are already first-class, and the fake provider
proves the built-in path; the remaining gap was that real providers
still leaked provider-specific config and runtime assumptions into core.
> - That coupling showed up in config normalization, secret persistence,
capabilities reporting, lease reconstruction, and the board UI form
fields.
> - As long as core knew about those provider-shaped details, shipping a
provider as a pure third-party plugin meant every new provider would
still require host changes.
> - This pull request generalizes the sandbox provider seam around
schema-driven plugin metadata and generic secret-ref handling.
> - The runtime and UI now consume provider metadata generically, so
core only special-cases the built-in fake provider while third-party
providers can live entirely in plugins.

## What Changed

- Added generic sandbox-provider capability metadata so plugin-backed
providers can expose `configSchema` through shared environment support
and the environments capabilities API.
- Reworked sandbox config normalization/persistence/runtime resolution
to handle schema-declared secret-ref fields generically, storing them as
Paperclip secrets and resolving them for probe/execute/release flows.
- Generalized plugin sandbox runtime handling so provider validation,
reusable-lease matching, lease reconstruction, and plugin worker calls
all operate on provider-agnostic config instead of provider-shaped
branches.
- Replaced hardcoded sandbox provider form fields in Company Settings
with schema-driven rendering and blocked agent environment selection
from the built-in fake provider.
- Added regression coverage for the generic seam across shared support
helpers plus environment config, probe, routes, runtime, and
sandbox-provider runtime tests.

## Verification

- `pnpm vitest --run packages/shared/src/environment-support.test.ts
server/src/__tests__/environment-config.test.ts
server/src/__tests__/environment-probe.test.ts
server/src/__tests__/environment-routes.test.ts
server/src/__tests__/environment-runtime.test.ts
server/src/__tests__/sandbox-provider-runtime.test.ts`
- `pnpm -r typecheck`

## Risks

- Plugin sandbox providers now depend more heavily on accurate
`configSchema` declarations; incorrect schemas can misclassify
secret-bearing fields or omit required config.
- Reusable lease matching is now metadata-driven for plugin-backed
providers, so providers that fail to persist stable metadata may
reprovision instead of resuming an existing lease.
- The UI form is now fully schema-driven for plugin-backed sandbox
providers; provider manifests without good defaults or descriptions may
produce a rougher operator experience.

## Model Used

- OpenAI Codex via `codex_local`
- Model ID: `gpt-5.4`
- Reasoning effort: `high`
- Context window observed in runtime session metadata: `258400` tokens
- Capabilities used: terminal tool execution, git, and local code/test
inspection

## 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
- [ ] 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

server/src/__tests__/environment-probe.test.ts

@@ -3,6 +3,7 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 const mockEnsureSshWorkspaceReady = vi.hoisted(() => vi.fn());
 const mockProbePluginEnvironmentDriver = vi.hoisted(() => vi.fn());
 const mockProbePluginSandboxProviderDriver = vi.hoisted(() => vi.fn());
+const mockResolvePluginSandboxProviderDriverByKey = vi.hoisted(() => vi.fn());
 
 vi.mock("@paperclipai/adapter-utils/ssh", () => ({
   ensureSshWorkspaceReady: mockEnsureSshWorkspaceReady,
@@ -11,6 +12,7 @@ vi.mock("@paperclipai/adapter-utils/ssh", () => ({
 vi.mock("../services/plugin-environment-driver.js", () => ({
   probePluginEnvironmentDriver: mockProbePluginEnvironmentDriver,
   probePluginSandboxProviderDriver: mockProbePluginSandboxProviderDriver,
+  resolvePluginSandboxProviderDriverByKey: mockResolvePluginSandboxProviderDriverByKey,
 }));
 
 import { probeEnvironment } from "../services/environment-probe.ts";
@@ -20,6 +22,8 @@ describe("probeEnvironment", () => {
     mockEnsureSshWorkspaceReady.mockReset();
     mockProbePluginEnvironmentDriver.mockReset();
     mockProbePluginSandboxProviderDriver.mockReset();
+    mockResolvePluginSandboxProviderDriverByKey.mockReset();
+    mockResolvePluginSandboxProviderDriverByKey.mockResolvedValue(null);
   });
 
   it("reports local environments as immediately available", async () => {