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/shared/src/index.ts

@@ -219,7 +219,12 @@ export type {
   Environment,
   EnvironmentLease,
   EnvironmentProbeResult,
+  FakeSandboxEnvironmentConfig,
   LocalEnvironmentConfig,
+  PluginSandboxEnvironmentConfig,
+  PluginEnvironmentConfig,
+  SandboxEnvironmentConfig,
+  SandboxEnvironmentProvider,
   SshEnvironmentConfig,
   FeedbackVote,
   FeedbackDataSharingPreference,
@@ -300,6 +305,10 @@ export type {
   WorkspaceOperationPhase,
   WorkspaceOperationStatus,
   WorkspaceRuntimeDesiredState,
+  WorkspaceRealizationRecord,
+  WorkspaceRealizationRequest,
+  WorkspaceRealizationSyncStrategy,
+  WorkspaceRealizationTransport,
   ExecutionWorkspaceStrategyType,
   ExecutionWorkspaceMode,
   ExecutionWorkspaceProviderType,
@@ -471,6 +480,7 @@ export type {
   PluginJobDeclaration,
   PluginWebhookDeclaration,
   PluginToolDeclaration,
+  PluginEnvironmentDriverDeclaration,
   PluginUiSlotDeclaration,
   PluginLauncherActionDeclaration,
   PluginLauncherRenderDeclaration,
@@ -542,17 +552,6 @@ export {
   isClosedIsolatedExecutionWorkspace,
 } from "./execution-workspace-guards.js";
 
-export {
-  adapterSupportsRemoteManagedEnvironments,
-  getAdapterEnvironmentSupport,
-  getEnvironmentCapabilities,
-  isEnvironmentDriverSupportedForAdapter,
-  supportedEnvironmentDriversForAdapter,
-  type AdapterEnvironmentSupport,
-  type EnvironmentCapabilities,
-  type EnvironmentSupportStatus,
-} from "./environment-support.js";
-
 export {
   instanceGeneralSettingsSchema,
   patchInstanceGeneralSettingsSchema,
@@ -824,6 +823,7 @@ export {
   pluginJobDeclarationSchema,
   pluginWebhookDeclarationSchema,
   pluginToolDeclarationSchema,
+  pluginEnvironmentDriverDeclarationSchema,
   pluginUiSlotDeclarationSchema,
   pluginLauncherActionDeclarationSchema,
   pluginLauncherRenderDeclarationSchema,
@@ -842,6 +842,7 @@ export {
   type PluginJobDeclarationInput,
   type PluginWebhookDeclarationInput,
   type PluginToolDeclarationInput,
+  type PluginEnvironmentDriverDeclarationInput,
   type PluginUiSlotDeclarationInput,
   type PluginLauncherActionDeclarationInput,
   type PluginLauncherRenderDeclarationInput,
@@ -926,3 +927,20 @@ export {
   type SecretsLocalEncryptedConfig,
   type ConfigMeta,
 } from "./config-schema.js";
+
+export {
+  adapterSupportsRemoteManagedEnvironments,
+  getEnvironmentCapabilities,
+  getAdapterEnvironmentSupport,
+  isEnvironmentDriverSupportedForAdapter,
+  isSandboxProviderSupportedForAdapter,
+  supportedEnvironmentDriversForAdapter,
+  supportedSandboxProvidersForAdapter,
+} from "./environment-support.js";
+
+export type {
+  AdapterEnvironmentSupport,
+  EnvironmentCapabilities,
+  EnvironmentProviderCapability,
+  EnvironmentSupportStatus,
+} from "./environment-support.js";