fix(acp-snapshot): stabilize /private prefix and spill paths in snapshots

macOS symlinks /tmp → /private/tmp; when the session header cwd lacks
the /private prefix but fs tools resolve symlinks, tool result paths
get the prefix and the cwd replacement never matches.  Strip the prefix
before cwd→{{cwd}} replacement so both forms normalize identically.

Also extend refreshFixtureReplacements to match spill paths by filename
suffix, preventing raw fixture churn when a refresh runs from a
different session directory.

packages/support/acp-snapshot/src/index.ts

@@ -33,6 +33,7 @@ export {
   type LaunchedAcpTestAgent,
 } from './launcher.ts'
 export {
+  extractSnapshotSpillPaths,
   normalizeSessionLog,
   normalizeStdout,
   scrubRequestHeaders,