Dashboard resume for missing historical working directories
- Persisted sessions remain visible and resumable when their recorded working directory no longer exists on the current host.
- Fleet and closed-session recovery now offer an explicit replacement-directory chooser, with strict canonical directory and access validation before runtime creation.
- Dashboard distinguishes immutable historical session provenance from the effective runtime directory without rewriting the original session header.
- Runtime-sensitive RPC behavior, new sessions, and message forks consistently use the effective runtime directory while preserving source history.
- Added regression coverage for symlink races, duplicate resume requests, fallback failures, persisted fork provenance, Dream paths, tab-title metadata, and parent/subagent recovery flows.
Implemented in PR 514, resolving issue 512.
