feat(server): verify existing-volume ephemeral enrollment (#18529)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Cloud web servers can now keep durable state in PostgreSQL and S3.
> - Existing servers still have files and a decision signing key on
their volume.
> - The volume must stay attached until every data owner verifies its
migration.
> - This pull request adds a dedicated maintenance process for that
transition.
> - Cloud can preserve the original key and remove the volume only after
a matching receipt.

## Linked Issues or Issue Description

Refs #18518 and #18526. Existing-volume enrollment is a required step in
the zero-downtime web deployment work. This is the Core maintenance
side; the Cloud transition is a separate private change.

## What Changed

- Package a standalone maintenance entry that starts no normal app
routes or execution workers.
- Coordinate the existing agent-file, run-evidence and skill migration
owners. Follow every page and verify shared bytes before final
eligibility.
- Require stopped writers. Refuse active execution, missing local bytes,
local workspaces and external instructions.
- Read the existing decision key without generating a replacement.
Preserve normal key creation for ordinary server startup.
- Require an attempt token for status and export. Export the original
key only after successful migration. Keep status and errors free of
secret material.
- Document the Cloud ownership boundary, deadline, retry and retention
requirements.

## Verification

- Full workspace typecheck and build passed before the review fixes. The
final server TypeScript compilation passed after correcting Drizzle's
optional predicate type.
- Production runtime bundling passed: 2,702 inputs, 122 chunks,
including the maintenance executable.
- The focused suite passed 21 cases using PostgreSQL and real
S3-compatible storage. It covers multi-page migration of 26 agents,
exact reads after deleting the predecessor home, corrupt-object
rejection, active/missing-data refusal, and the bundled plain-Node
process with authenticated export and clean SIGTERM exit.
- Regression cases refuse unsaved terminal working copies, require
explicit stopped/cleaned receipts for unavailable copies, clean 101
expired traces through the owning service, and enforce the deadline
after migration succeeds.
- Initial integration attempts exposed fixture cleanup ordering
problems; corrected runs pass. CI also caught a Drizzle predicate type
error, now fixed and compiled locally.
- All repository CI gates passed on a4db955403, including typecheck,
build, browser suites and serialized tests. Greptile reviewed that head
at 5/5; all four review threads are resolved. Hosted enrollment and
active-agent continuity remain separate acceptance work.

## Risks

- This is an explicit maintenance entry. The caller must confirm all
prior deployments stopped and keep normal tenant routing closed through
detachment.
- A timeout blocks export but an owner may finish its current bounded
operation before observing cancellation. Cloud must confirm process exit
before detaching. SIGTERM has a ten-second hard exit deadline.
- The authenticated export contains the original signing key. The Cloud
caller must put it only in the stack vault and must not log the body.
- Local-workspace and external-instruction migrations remain their
existing owners' responsibility. Enrollment fails closed until those
data are eligible.
- No database schema changes. Ordinary server startup is unchanged
unless this maintenance entry is explicitly launched.

## Model Used

OpenAI Codex, GPT-6. Tool use, code execution and repository review. The
runtime does not expose a more specific model ID or context-window size.

## 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

@@ -80,6 +80,15 @@ describe("bundleRuntime", () => {
     expect(JSON.parse(stdout)).toEqual({ asset: "asset beside its source", phase: "--phase=eligibility" });
   });
 
+  it("packages standalone enrollment without starting the app", async () => {
+    const ephemeralMigrationEntry = path.join(root, "server", "dist", "migration.js");
+    write(ephemeralMigrationEntry, 'import { values } from "@paperclipai/fixture"; console.log(JSON.stringify({ migrated: values.doubled }));');
+    const outdir = path.join(root, "server", "dist-runtime");
+    await bundleRuntime({ entry: path.join(root, "server", "dist", "index.js"), outdir, ephemeralMigrationEntry });
+    const { stdout } = await execFileAsync(process.execPath, [path.join(outdir, "ephemeral-migration.mjs")], { timeout: 30_000 });
+    expect(JSON.parse(stdout)).toEqual({ migrated: 42 });
+  });
+
   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 });