fix: recover from stale deleted workspaces

[dobby-d-elf]

Summary

• fix workspace fallback logic so WebUI uses the live DEFAULT_WORKSPACE instead of a stale import-time snapshot
• recover chat sends when an old session points at a deleted workspace and the browser did not explicitly choose that path
• add regression tests for stale default-workspace fallback and missing-session-workspace recovery

What this fixes

This fixes live Hermes WebUI chats getting stuck with errors like:

Error: Path does not exist: /Users/agent/.hermes/webui-test-.../test-workspace

Root cause:

• a deleted test workspace path could remain baked into fallback workspace state
• /api/workspaces could keep serving that stale path when no saved workspace file existed
• old sessions saved with that dead workspace would fail on the next send instead of recovering

With this change:

• workspace fallback uses the current runtime default workspace
• stale implicit session workspaces are repaired to the current valid workspace during chat start

Test Plan

• Tests added to reproduce this issue in code and passing
• Also human verified it recovered broken sessions which had this issue.