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

ui/src/api/activity.ts

@@ -25,6 +25,25 @@ export interface RunForIssue {
   continuationAttempt?: number;
   lastUsefulActionAt?: string | null;
   nextAction?: string | null;
+  contextSnapshot?: Record<string, unknown> | null;
+  environment?: {
+    id: string;
+    name: string;
+    driver: string;
+  } | null;
+  environmentLease?: {
+    id: string;
+    status: string;
+    leasePolicy: string;
+    provider: string | null;
+    providerLeaseId: string | null;
+    executionWorkspaceId: string | null;
+    workspacePath: string | null;
+    failureReason: string | null;
+    cleanupStatus: string | null;
+    acquiredAt: string | Date;
+    releasedAt: string | Date | null;
+  } | null;
 }
 
 export interface IssueForRun {

ui/src/api/environments.ts

@@ -9,14 +9,14 @@ export const environmentsApi = {
   create: (companyId: string, body: {
     name: string;
     description?: string | null;
-    driver: "local" | "ssh";
+    driver: "local" | "ssh" | "sandbox" | "plugin";
     config?: Record<string, unknown>;
     metadata?: Record<string, unknown> | null;
   }) => api.post<Environment>(`/companies/${companyId}/environments`, body),
   update: (environmentId: string, body: {
     name?: string;
     description?: string | null;
-    driver?: "local" | "ssh";
+    driver?: "local" | "ssh" | "sandbox" | "plugin";
     status?: "active" | "archived";
     config?: Record<string, unknown>;
     metadata?: Record<string, unknown> | null;
@@ -24,8 +24,8 @@ export const environmentsApi = {
   probe: (environmentId: string) => api.post<EnvironmentProbeResult>(`/environments/${environmentId}/probe`, {}),
   probeConfig: (companyId: string, body: {
     name?: string;
+    driver: "local" | "ssh" | "sandbox" | "plugin";
     description?: string | null;
-    driver: "local" | "ssh";
     config?: Record<string, unknown>;
     metadata?: Record<string, unknown> | null;
   }) => api.post<EnvironmentProbeResult>(`/companies/${companyId}/environments/probe-config`, body),