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

packages/plugins/sdk/src/index.ts

@@ -50,7 +50,7 @@
 // ---------------------------------------------------------------------------
 
 export { definePlugin } from "./define-plugin.js";
-export { createTestHarness } from "./testing.js";
+export { createTestHarness, createEnvironmentTestHarness, createFakeEnvironmentDriver, filterEnvironmentEvents, assertEnvironmentEventOrder, assertLeaseLifecycle, assertWorkspaceRealizationLifecycle, assertExecutionLifecycle, assertEnvironmentError } from "./testing.js";
 export { createPluginBundlerPresets } from "./bundlers.js";
 export { startPluginDevServer, getUiBuildSnapshot } from "./dev-server.js";
 export { startWorkerRpcHost, runWorker } from "./worker-rpc-host.js";
@@ -102,6 +102,10 @@ export type {
   TestHarness,
   TestHarnessOptions,
   TestHarnessLogEntry,
+  EnvironmentTestHarness,
+  EnvironmentTestHarnessOptions,
+  EnvironmentEventRecord,
+  FakeEnvironmentDriverOptions,
 } from "./testing.js";
 export type {
   PluginBundlerPresetInput,
@@ -142,6 +146,21 @@ export type {
   GetDataParams,
   PerformActionParams,
   ExecuteToolParams,
+  PluginEnvironmentDiagnostic,
+  PluginEnvironmentDriverBaseParams,
+  PluginEnvironmentValidateConfigParams,
+  PluginEnvironmentValidationResult,
+  PluginEnvironmentProbeParams,
+  PluginEnvironmentProbeResult,
+  PluginEnvironmentLease,
+  PluginEnvironmentAcquireLeaseParams,
+  PluginEnvironmentResumeLeaseParams,
+  PluginEnvironmentReleaseLeaseParams,
+  PluginEnvironmentDestroyLeaseParams,
+  PluginEnvironmentRealizeWorkspaceParams,
+  PluginEnvironmentRealizeWorkspaceResult,
+  PluginEnvironmentExecuteParams,
+  PluginEnvironmentExecuteResult,
   PluginModalBoundsRequest,
   PluginRenderCloseEvent,
   PluginLauncherRenderContextSnapshot,
@@ -235,6 +254,7 @@ export type {
   PluginJobDeclaration,
   PluginWebhookDeclaration,
   PluginToolDeclaration,
+  PluginEnvironmentDriverDeclaration,
   PluginUiSlotDeclaration,
   PluginUiDeclaration,
   PluginLauncherActionDeclaration,