fix(workspaces): preserve snapshots beside runtime sockets (#19159)

## Thinking Path

> - A workspace can contain saved source files alongside transient
runtime IPC endpoints.
> - The portable archive supports files, directories and confined
symlinks; treating a Unix socket as a fatal entry prevents every file
beside it from being saved.
> - Keep the existing archive owner and validation. Record socket paths
as exclusions, leaving the source endpoint and adjacent work intact.

## Linked Issues or Issue Description

**What happened?**

Retained workspace recovery repeatedly fails with
`remote_workspace_archive_unsupported_entry` when the source tree
contains a Unix socket, preventing adjacent source files from being
saved. A real socket reproduces the error in local capture and the
serialized sandbox helper.

**Expected behavior**

Save regular workspace files, record runtime socket paths as exclusions,
and leave the source endpoints and new file edits intact.

**Steps to reproduce**

1. Create a temporary workspace with an uncommitted text file.
2. Open a Unix domain socket in that directory with a Node server.
3. Capture the workspace through the local archive function or generated
sandbox helper. Before this fix both captures fail; afterward the saved
file restores and the socket exclusion is recorded.

**Paperclip version or commit**

Reproduced on `6fa54d55779295d90c455389cebdd3da96607d20` before this
change. The socket fixture establishes this failure class; it does not
establish the entry type behind every historical unsupported-entry
occurrence.

## What Changed

- Exclude Unix domain socket endpoints from portable workspace captures
and include their paths in the existing exclusion manifest.
- Keep source sockets in place. Preserve normal files and the existing
rejection of named pipes, device entries and unsafe links.
- Exercise local and serialized capture, an unchanged source endpoint,
restoration of adjacent uncommitted work, FIFO rejection, and a socket
becoming a real file during capture. A changed source still fails safely
and retains that new file.
- Document the portable snapshot behavior.

## Verification

- Red regression: both local and serialized captures failed with
`remote_workspace_archive_unsupported_entry` before the change.
- `pnpm exec vitest run
server/src/services/remote-workspace-snapshot-archive.test.ts`: **43
passed** using real filesystem sockets, pipes, Git repositories and
archives.
- `pnpm -r typecheck` and `pnpm build`: passed.
- Module boundaries and `git diff --check`: passed. Added source and PR
text were scanned for credentials, private incident identifiers and
personal data before push.
- Snapshot storage and save-concurrency consumer suites pass **23
tests**. Their first local runs exposed missing workspace SDK build
links and unhydrated macOS embedded-Postgres library symlinks in the new
checkout. Repairing those prerequisites made the unchanged consumer
suites pass; the earlier failures remain recorded and no assertion was
weakened.
- Greptile reviewed `e313565a5088abce448f11b29982462aa338a692` at
**5/5** with no unresolved threads. All current-head CI checks are green
and the branch is conflict-free. Hosted recovery needs a qualified
deployment and original-task verification.

## Risks

Socket endpoints represent runtime IPC and are not recreated by archive
restore. Their paths are visible in the existing exclusion manifest.
This adds no archive entry type, subprocess, recurring work, provider
call or dependency. It does not alter source authorization, stop
receipts, ordinary file validation, restore safety or recovery budgets.
Unix socket/FIFO fixtures are skipped on Windows; existing portable
archive coverage still runs there.

## Model Used

OpenAI GPT-6.1 Sol (`gpt-6.1-sol`) through Codex, xhigh reasoning. Used
repository tools, code execution and real local filesystem/process
fixtures.

## Checklist

- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] Added focused behavioral and preservation regression coverage
- [x] Updated the execution contract and documented limits
- [x] Typecheck and build pass locally
- [x] Scanned the diff and description before push
- [x] Current-head CI is fully green
- [x] Greptile is 5/5 with no unresolved comments

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

doc/execution-semantics.md

@@ -874,6 +874,12 @@ Warm sandbox reuse must match the current host Git tip and branch as well as the
 file snapshot and saved stamp, including managed nested repositories. A history
 or branch mismatch restages the host before the next run begins.
 
+Portable workspace archives exclude Unix domain socket endpoints and record their
+paths in the exclusion manifest. Capture leaves the source endpoints in place;
+restore does not recreate runtime IPC. Named pipes and device entries remain
+unsupported, while regular files, directories and confined symlinks retain their
+existing validation.
+
 Disk-backed native sandbox workspace snapshots run in at most two worker threads per
 controller process. Each capture retains the same file hashes, modes, symlinks,
 exclusions and ignored-path manifest as an inline capture. A worker seals its