Add sandbox environment support (#4415)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - The environment/runtime layer decides where agent work executes and
how the control plane reaches those runtimes.
> - Today Paperclip can run locally and over SSH, but sandboxed
execution needs a first-class environment model instead of one-off
adapter behavior.
> - We also want sandbox providers to be pluggable so the core does not
hardcode every provider implementation.
> - This branch adds the Sandbox environment path, the provider
contract, and a deterministic fake provider plugin.
> - That required synchronized changes across shared contracts, plugin
SDK surfaces, server runtime orchestration, and the UI
environment/workspace flows.
> - The result is that sandbox execution becomes a core control-plane
capability while keeping provider implementations extensible and
testable.

## What Changed

- Added sandbox runtime support to the environment execution path,
including runtime URL discovery, sandbox execution targeting,
orchestration, and heartbeat integration.
- Added plugin-provider support for sandbox environments so providers
can be supplied via plugins instead of hardcoded server logic.
- Added the fake sandbox provider plugin with deterministic behavior
suitable for local and automated testing.
- Updated shared types, validators, plugin protocol definitions, and SDK
helpers to carry sandbox provider and workspace-runtime contracts across
package boundaries.
- Updated server routes and services so companies can create sandbox
environments, select them for work, and execute work through the sandbox
runtime path.
- Updated the UI environment and workspace surfaces to expose sandbox
environment configuration and selection.
- Added test coverage for sandbox runtime behavior, provider seams,
environment route guards, orchestration, and the fake provider plugin.

## Verification

- Ran locally before the final fixture-only scrub:
  - `pnpm -r typecheck`
  - `pnpm test:run`
  - `pnpm build`
- Ran locally after the final scrub amend:
  - `pnpm vitest run server/src/__tests__/runtime-api.test.ts`
- Reviewer spot checks:
  - create a sandbox environment backed by the fake provider plugin
  - run work through that environment
- confirm sandbox provider execution does not inherit host secrets
implicitly

## Risks

- This touches shared contracts, plugin SDK plumbing, server runtime
orchestration, and UI environment/workspace flows, so regressions would
likely show up as cross-layer mismatches rather than isolated type
errors.
- Runtime URL discovery and sandbox callback selection are sensitive to
host/bind configuration; if that logic is wrong, sandbox-backed
callbacks may fail even when execution succeeds.
- The fake provider plugin is intentionally deterministic and
test-oriented; future providers may expose capability gaps that this
branch does not yet cover.

## Model Used

- OpenAI Codex coding agent on a GPT-5-class backend in the
Paperclip/Codex harness. Exact backend model ID is not exposed
in-session. Tool-assisted workflow with shell execution, file editing,
git history inspection, and local test execution.

## 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/services/environment-probe.ts

@@ -6,11 +6,14 @@ import {
   type ParsedEnvironmentConfig,
 } from "./environment-config.js";
 import os from "node:os";
+import { isBuiltinSandboxProvider, probeSandboxProvider } from "./sandbox-provider-runtime.js";
+import { probePluginEnvironmentDriver, probePluginSandboxProviderDriver } from "./plugin-environment-driver.js";
+import type { PluginWorkerManager } from "./plugin-worker-manager.js";
 
 export async function probeEnvironment(
   db: Db,
   environment: Environment,
-  options: { resolvedConfig?: ParsedEnvironmentConfig } = {},
+  options: { pluginWorkerManager?: PluginWorkerManager; resolvedConfig?: ParsedEnvironmentConfig } = {},
 ): Promise<EnvironmentProbeResult> {
   const parsed = options.resolvedConfig ?? await resolveEnvironmentDriverConfigForRuntime(db, environment.companyId, environment);
 
@@ -26,6 +29,51 @@ export async function probeEnvironment(
     };
   }
 
+  if (parsed.driver === "sandbox") {
+    if (!isBuiltinSandboxProvider(parsed.config.provider)) {
+      if (!options.pluginWorkerManager) {
+        return {
+          ok: false,
+          driver: "sandbox",
+          summary: `Sandbox provider "${parsed.config.provider}" requires a running provider plugin.`,
+          details: {
+            provider: parsed.config.provider,
+          },
+        };
+      }
+      return await probePluginSandboxProviderDriver({
+        db,
+        workerManager: options.pluginWorkerManager,
+        companyId: environment.companyId,
+        environmentId: environment.id,
+        provider: parsed.config.provider,
+        config: parsed.config as unknown as Record<string, unknown>,
+      });
+    }
+    return await probeSandboxProvider(parsed.config);
+  }
+
+  if (parsed.driver === "plugin") {
+    if (!options.pluginWorkerManager) {
+      return {
+        ok: false,
+        driver: "plugin",
+        summary: `Plugin environment probes require a plugin worker manager for "${parsed.config.pluginKey}:${parsed.config.driverKey}".`,
+        details: {
+          pluginKey: parsed.config.pluginKey,
+          driverKey: parsed.config.driverKey,
+        },
+      };
+    }
+    return await probePluginEnvironmentDriver({
+      db,
+      workerManager: options.pluginWorkerManager,
+      companyId: environment.companyId,
+      environmentId: environment.id,
+      config: parsed.config,
+    });
+  }
+
   try {
     const { remoteCwd } = await ensureSshWorkspaceReady(parsed.config);