feat: ghost snapshot v2 (#8055)

This PR updates ghost snapshotting to avoid capturing oversized
untracked artifacts while keeping undo safe. Snapshot creation now
builds a temporary index from `git status --porcelain=2 -z`, writes a
tree and detached commit without touching refs, and records any ignored
large files/dirs in the snapshot report. Undo uses that metadata to
preserve large local artifacts while still cleaning up new transient
files.

codex-rs/Cargo.lock

@@ -44,7 +44,7 @@ dependencies = [
  "bytestring",
  "derive_more 2.0.1",
  "encoding_rs",
- "foldhash",
+ "foldhash 0.1.5",
  "futures-core",
  "http 0.2.12",
  "httparse",
@@ -139,7 +139,7 @@ dependencies = [
  "cfg-if",
  "derive_more 2.0.1",
  "encoding_rs",
- "foldhash",
+ "foldhash 0.1.5",
  "futures-core",
  "futures-util",
  "impl-more",
@@ -154,7 +154,7 @@ dependencies = [
  "serde_json",
  "serde_urlencoded",
  "smallvec",
- "socket2 0.6.0",
+ "socket2 0.6.1",
  "time",
  "tracing",
  "url",