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/adapter-utils/src/execution-target-sandbox.test.ts

@@ -0,0 +1,96 @@
+import { describe, expect, it, vi } from "vitest";
+
+import {
+  adapterExecutionTargetSessionIdentity,
+  adapterExecutionTargetToRemoteSpec,
+  runAdapterExecutionTargetProcess,
+  runAdapterExecutionTargetShellCommand,
+  type AdapterSandboxExecutionTarget,
+} from "./execution-target.js";
+
+describe("sandbox adapter execution targets", () => {
+  it("executes through the provider-neutral runner without a remote spec", async () => {
+    const runner = {
+      execute: vi.fn(async () => ({
+        exitCode: 0,
+        signal: null,
+        timedOut: false,
+        stdout: "ok\n",
+        stderr: "",
+        pid: null,
+        startedAt: new Date().toISOString(),
+      })),
+    };
+    const target: AdapterSandboxExecutionTarget = {
+      kind: "remote",
+      transport: "sandbox",
+      providerKey: "acme-sandbox",
+      environmentId: "env-1",
+      leaseId: "lease-1",
+      remoteCwd: "/workspace",
+      timeoutMs: 30_000,
+      runner,
+    };
+
+    expect(adapterExecutionTargetToRemoteSpec(target)).toBeNull();
+
+    const result = await runAdapterExecutionTargetProcess("run-1", target, "agent-cli", ["--json"], {
+      cwd: "/local/workspace",
+      env: { TOKEN: "token" },
+      stdin: "prompt",
+      timeoutSec: 5,
+      graceSec: 1,
+      onLog: async () => {},
+    });
+
+    expect(result.stdout).toBe("ok\n");
+    expect(runner.execute).toHaveBeenCalledWith(expect.objectContaining({
+      command: "agent-cli",
+      args: ["--json"],
+      cwd: "/workspace",
+      env: { TOKEN: "token" },
+      stdin: "prompt",
+      timeoutMs: 5000,
+    }));
+    expect(adapterExecutionTargetSessionIdentity(target)).toEqual({
+      transport: "sandbox",
+      providerKey: "acme-sandbox",
+      environmentId: "env-1",
+      leaseId: "lease-1",
+      remoteCwd: "/workspace",
+    });
+  });
+
+  it("runs shell commands through the same runner", async () => {
+    const runner = {
+      execute: vi.fn(async () => ({
+        exitCode: 0,
+        signal: null,
+        timedOut: false,
+        stdout: "/home/sandbox",
+        stderr: "",
+        pid: null,
+        startedAt: new Date().toISOString(),
+      })),
+    };
+    const target: AdapterSandboxExecutionTarget = {
+      kind: "remote",
+      transport: "sandbox",
+      remoteCwd: "/workspace",
+      runner,
+    };
+
+    await runAdapterExecutionTargetShellCommand("run-2", target, 'printf %s "$HOME"', {
+      cwd: "/local/workspace",
+      env: {},
+      timeoutSec: 7,
+    });
+
+    expect(runner.execute).toHaveBeenCalledWith(expect.objectContaining({
+      command: "sh",
+      args: ["-lc", 'printf %s "$HOME"'],
+      cwd: "/workspace",
+      timeoutMs: 7000,
+    }));
+  });
+});