feat(server): support volume-free web server handoff (#18518)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Its web server also owns controller admission and user commands.
> - A rolling deployment starts a replacement before the old server
exits.
> - Shared database state alone does not preserve local agent files,
browser chunks, or in-process commands.
> - This pull request adds an opt-in ephemeral server contract and a
local overlap test.
> - The benefit is a web server that can relinquish traffic and local
files without losing acknowledged work.

## Linked Issues or Issue Description

Refs #18403, #18327, #18335, #18341.

**Current behavior**

The control-plane web server still has local agent-file authority and
local command targets. A new container can miss saved bytes or send a
command to the wrong process. An existing browser tab also needs chunks
from its original build.

**Proposed behavior**

Eligible servers share only PostgreSQL, object storage and stable
secrets. A replacement waits for readiness. The old server drains
admitted requests, keeps owner commands available through handoff, and
releases controller leases. Explicit maintenance adopts existing files
before the volume is removed.

**Reason and benefit**

This is the application prerequisite for rolling app-only deployments.
Schema migrations and permanent multi-replica operation are outside this
change.

## What Changed

- Store agent-file manifests in PostgreSQL and immutable bytes in S3.
Treat local roots as disposable projections. Preserve existing
permission checks and edit conflicts.
- Add an explicit migration, eligibility and object-retention command to
the production runtime bundle.
- Add `/api/ready`, admitted-request drain, browser restart signaling
and retained immutable UI chunks.
- Carry transaction-aware work hints across controllers during handoff
and active execution. Keep listeners closed when idle.
- Deliver durable Stop and runtime responses to the execution owner.
Recheck request lifecycle and access before response dispatch.
- Reject local-only configuration in ephemeral mode. Keep the feature
disabled for existing installations by default.
- Add an isolated PostgreSQL/S3/proxy/browser deployment harness with
independent server homes and retained failure evidence.

## Verification

- All CI checks pass on `ce17b95019d3cf1cabd357d19b6d37f96782cc5c`:
workspace typecheck, build, Runner checks, server/general tests and
browser shards. Greptile reviewed the runtime changes at 5/5 with all
findings resolved; the final commit updates maps and the plan.
- Local targeted checks passed: coordinator 27; Stop/recovery 33;
PostgreSQL command delivery 6; route/workspace regressions 8; bootstrap
Stop 1; restart recovery 75. PostgreSQL command tests simulate the
provider edge.
- Local browser proof passed 91 reads and 19 saves; reconnect took
1,013ms. The tab opened Audit from S3 after deletion of A's whole
home/build. This uses compiled workspace code and local-trusted auth,
not released containers or live agents.
- Local workspace typecheck/build passed; current server and runtime
bundle passed. The earlier broad local suite was stopped after three
hours and is not a passing receipt. Its retained failures and isolated
reruns are documented in the plan.
- Stacked #18526 adds stricter graceful-exit proof, an unready
replacement control, live Runner qualification and a Stop handoff
correction. Hosted enablement remains gated on those results and Cloud
#1096. No hosted stack has been enabled.

## Risks

- Apply migration 0422 before enabling the feature. Existing data must
pass each owner's migration and eligibility checks. Do not discard a
volume based on a single migration page.
- Durable manifests remain authoritative after the opt-in is disabled.
Rolling back to a binary that ignores them is unsupported.
- Cold directory restoration and complete shutdown timing still need
qualification before enabling a hosted stack.
- Retained UI assets must cover the supported browser and rollback
window. Cleanup is explicit; no new idle maintenance polling is
introduced.
- This change does not claim production zero downtime or authorize a
fleet-wide enablement.

## Model Used

OpenAI Codex, GPT-6 family. The exact serving model ID and
context-window limit are not exposed in this session. Used reasoning,
code editing, shell execution and browser automation.

## 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 searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

server/scripts/bundle-runtime.test.mjs

@@ -71,6 +71,15 @@ describe("bundleRuntime", () => {
     rmSync(root, { recursive: true, force: true });
   });
 
+  it("packages a maintenance entry without starting the application or needing a TypeScript loader", async () => {
+    const maintenanceEntry = path.join(root, "server", "dist", "maintenance.js");
+    write(maintenanceEntry, 'import { asset } from "@paperclipai/fixture"; console.log(JSON.stringify({ asset, phase: process.argv.at(-1) }));');
+    const outdir = path.join(root, "server", "dist-runtime");
+    await bundleRuntime({ entry: path.join(root, "server", "dist", "index.js"), outdir, maintenanceEntry });
+    const { stdout } = await execFileAsync(process.execPath, [path.join(outdir, "agent-files.mjs"), "--phase=eligibility"], { timeout: 30_000 });
+    expect(JSON.parse(stdout)).toEqual({ asset: "asset beside its source", phase: "--phase=eligibility" });
+  });
+
   it("bundles workspace TypeScript and runs with plain node", async () => {
     const outdir = path.join(root, "server", "dist-runtime");
     const summary = await bundleRuntime({ entry: path.join(root, "server", "dist", "index.js"), outdir });