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/index.ts

@@ -36,6 +36,8 @@ import {
   routineService,
 } from "./services/index.js";
 import { createFeedbackTraceShareClientFromConfig } from "./services/feedback-share-client.js";
+import { buildRuntimeApiCandidateUrls, choosePrimaryRuntimeApiUrl } from "./runtime-api.js";
+import { createPluginWorkerManager } from "./services/plugin-worker-manager.js";
 import { createStorageServiceFromConfig } from "./storage/index.js";
 import { printStartupBanner } from "./startup-banner.js";
 import { getBoardClaimWarningUrl, initializeBoardClaimChallenge } from "./board-claim.js";
@@ -590,6 +592,7 @@ export async function startServer(): Promise<StartedServer> {
       databaseBackupInFlight = false;
     }
   };
+  const pluginWorkerManager = createPluginWorkerManager();
   const app = await createApp(db as any, {
     uiMode,
     serverPort: listenPort,
@@ -613,6 +616,7 @@ export async function startServer(): Promise<StartedServer> {
     pluginMigrationDb: pluginMigrationDb as any,
     betterAuthHandler,
     resolveSession,
+    pluginWorkerManager,
   });
   const server = createServer(app as unknown as Parameters<typeof createServer>[0]);
 
@@ -627,15 +631,24 @@ export async function startServer(): Promise<StartedServer> {
   }
   
   const runtimeListenHost = config.host;
-  const runtimeApiHost =
-    runtimeListenHost === "0.0.0.0" || runtimeListenHost === "::"
-      ? "localhost"
-      : runtimeListenHost;
+  const runtimeApiUrl = choosePrimaryRuntimeApiUrl({
+    authPublicBaseUrl: config.authPublicBaseUrl ?? null,
+    allowedHostnames: config.allowedHostnames,
+    bindHost: runtimeListenHost,
+    port: listenPort,
+  });
+  const runtimeApiCandidates = buildRuntimeApiCandidateUrls({
+    authPublicBaseUrl: config.authPublicBaseUrl ?? null,
+    allowedHostnames: config.allowedHostnames,
+    bindHost: runtimeListenHost,
+    port: listenPort,
+  });
+  const configuredApiUrl = process.env.PAPERCLIP_API_URL?.trim() || runtimeApiUrl;
   process.env.PAPERCLIP_LISTEN_HOST = runtimeListenHost;
   process.env.PAPERCLIP_LISTEN_PORT = String(listenPort);
-  if (!process.env.PAPERCLIP_API_URL) {
-    process.env.PAPERCLIP_API_URL = `http://${runtimeApiHost}:${listenPort}`;
-  }
+  process.env.PAPERCLIP_RUNTIME_API_URL = runtimeApiUrl;
+  process.env.PAPERCLIP_RUNTIME_API_CANDIDATES_JSON = JSON.stringify(runtimeApiCandidates);
+  process.env.PAPERCLIP_API_URL = configuredApiUrl;
   
   setupLiveEventsWebSocketServer(server, db as any, {
     deploymentMode: config.deploymentMode,
@@ -656,8 +669,8 @@ export async function startServer(): Promise<StartedServer> {
     });
   
   if (config.heartbeatSchedulerEnabled) {
-    const heartbeat = heartbeatService(db as any);
-    const routines = routineService(db as any);
+    const heartbeat = heartbeatService(db as any, { pluginWorkerManager });
+    const routines = routineService(db as any, { pluginWorkerManager });
   
     // Reap orphaned running runs at startup while in-memory execution state is empty,
     // then resume any persisted queued runs that were waiting on the previous process.
@@ -860,7 +873,7 @@ export async function startServer(): Promise<StartedServer> {
     server,
     host: config.host,
     listenPort,
-    apiUrl: process.env.PAPERCLIP_API_URL!,
+    apiUrl: configuredApiUrl,
     databaseUrl: activeDatabaseConnectionString,
   };
 }