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/components/CommentThread.tsx

@@ -40,6 +40,22 @@ interface LinkedRunItem {
   agentId: string;
   createdAt: Date | string;
   startedAt: Date | string | 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;
+  } | null;
   finishedAt?: Date | string | null;
 }
 
@@ -119,6 +135,16 @@ function clearDraft(draftKey: string) {
   }
 }
 
+function BreakablePath({ text }: { text: string }) {
+  const parts: React.ReactNode[] = [];
+  const segments = text.split(/(?<=[\/-])/);
+  for (let i = 0; i < segments.length; i++) {
+    if (i > 0) parts.push(<wbr key={i} />);
+    parts.push(segments[i]);
+  }
+  return <>{parts}</>;
+}
+
 function parseReassignment(target: string): CommentReassignment | null {
   if (!target || target === "__none__") {
     return { assigneeAgentId: null, assigneeUserId: null };
@@ -611,6 +637,40 @@ const TimelineList = memo(function TimelineList({
                   </a>
                 </div>
               </div>
+              {run.environment || run.environmentLease ? (
+                <div className="mt-2 flex flex-wrap gap-x-3 gap-y-1 text-[11px] text-muted-foreground">
+                  {run.environment ? (
+                    <span>
+                      Environment <span className="text-foreground">{run.environment.name}</span>
+                      <span> · {run.environment.driver}</span>
+                    </span>
+                  ) : null}
+                  {run.environmentLease?.provider ? (
+                    <span>
+                      Provider <span className="text-foreground">{run.environmentLease.provider}</span>
+                    </span>
+                  ) : null}
+                  {run.environmentLease ? (
+                    <span>
+                      Lease{" "}
+                      <span className="font-mono text-foreground">
+                        {run.environmentLease.id.slice(0, 8)}
+                      </span>
+                      <span> · {run.environmentLease.status}</span>
+                    </span>
+                  ) : null}
+                  {run.environmentLease?.workspacePath ? (
+                    <span className="min-w-0 font-mono" style={{ overflowWrap: "anywhere" }}>
+                      <BreakablePath text={run.environmentLease.workspacePath} />
+                    </span>
+                  ) : null}
+                  {run.environmentLease?.failureReason ? (
+                    <span className="text-destructive">
+                      Failure: {run.environmentLease.failureReason}
+                    </span>
+                  ) : null}
+                </div>
+              ) : null}
             </div>
           );
         }