(remote): narrow the backslash root, keep //server paths normalised (follow-up #912) #914

Follow-up of #912 (#909), review minors, low priority.

1. Single-backslash root over-merges escape tokens. remote-index.js ~327: the new \ alternative (any single backslash) applies at every anchor, so unexpected token \n and unexpected token \t both become unexpected token <path>, and esc \u001b[0m becomes esc <path>. Two parse errors differing only by an escape token share one signature. Require a path character and a second separator after the backslash (e.g. \(?=[^\s\]*\)), or keep \\ and handle \x\… only after =; or document the limit in .ai/contexts/session-cache.md.
2. Unquoted forward-slash UNC paths are no longer normalised. The (?!\/) guard applies at every anchor, not only after :: //server/share/a failed stayed <path> failed on (remote): anchor warning path normalisation, cap signatures, notify on deleted run snapshots (#889, #904) #908 and now stays raw, as does key=//server/share/a. Apply the guard only after : (e.g. (?<!:)\/|(?<=:)\/(?!\/)), or document it.
3. Suggestion: workflowTitles keeps run ids of deleted snapshots (main.js ~3629-3633), pruned only on folder events; ENOENT events for a folder removed in the same flush are re-inserted after the prune. One short string per deleted run, effectively bounded. Optionally skip the set when the folder no longer exists.