fix(desktop): stabilize snapshot sidecar lifecycle - #31283

TLDR;

• Snapshot capture no longer gets stuck behind a stale internal Git index lock.
• Early Git failures no longer leave a late pipe error that can terminate the local Desktop server.
• Desktop no longer keeps a terminated local server marked as active.

Problem

OpenCode snapshots use Git with a NUL-delimited path list written to child-process stdin. If the snapshot repository contains a stale index.lock, Git exits before consuming that input. On Windows, the outstanding overlapped pipe write can then complete as write EOF; if stdin lifecycle finishes after process completion, that late socket error can terminate the Desktop sidecar instead of preserving Git's real exit status and lock diagnostic.

The shared snapshot index also means the stale lock continues blocking later captures. This change isolates each capture in a scoped alternate index, persists its baseline as an immutable Git tree ref, and settles child stdin before exposing process completion.

Headless repro

Run with Node 24 and Git for Windows. This uses the same git add --pathspec-from-file=- --pathspec-file-nul shape as snapshot capture:

// repro.mjs
import { spawn, spawnSync } from "node:child_process"
import { mkdirSync, rmSync, writeFileSync } from "node:fs"
import { join } from "node:path"

const root = join(import.meta.dirname, "snapshot-eof-repro")
const worktree = join(root, "worktree")
const gitdir = join(root, "snapshot.git")
rmSync(root, { recursive: true, force: true })
mkdirSync(worktree, { recursive: true })
spawnSync("git", ["init", "--bare", gitdir])
writeFileSync(join(gitdir, "index.lock"), "")

const input = Buffer.alloc(4 * 1024 * 1024, 0x61)
for (let i = 255; i < input.length; i += 256) input[i] = 0

const child = spawn("git", [
  "--git-dir", gitdir,
  "--work-tree", worktree,
  "add", "--all",
  "--pathspec-from-file=-",
  "--pathspec-file-nul",
], { stdio: ["overlapped", "pipe", "pipe"], windowsHide: true })

child.stdin.end(input)

node repro.mjs

Before this change, the process terminates with the same failure observed in Desktop diagnostics:

Error: write EOF
    at WriteWrap.onWriteComplete [as oncomplete]
Emitted 'error' event on Socket instance
code: 'EOF'
syscall: 'write'

Changelog

Fixed

• OpenCode Desktop no longer loses snapshot capture when an old internal snapshot lock remains on disk.
• Files already tracked by snapshots remain tracked after migration, including files that later grow beyond the new-file size limit.
• The local Desktop server no longer terminates from a delayed input-pipe error after Git exits early.
• Git failures retain their original exit status and diagnostic output instead of being replaced by a secondary pipe error.
• Successful commands remain successful when they intentionally stop reading input before it is exhausted.
• An unexpectedly terminated local server is no longer kept as the current Desktop server connection.
• A previous local server process exiting after replacement no longer clears the newer active server.

Improved

• Snapshot capture now uses a temporary scoped Git index and a Git-managed baseline ref while preserving existing snapshot retention, pruning, serialization, restore, and revert behavior.
• Intentional Desktop shutdown is distinguished from an unexpected local server exit in diagnostics.