Support importing/backfilling local CLI sessions into the session registry

Sessions run via the Claude Code CLI (claude in a terminal) are stored locally under ~/.claude/projects/<sanitized-cwd>/*.jsonl. The hosted app/harness session list ("recents") appears to track a separate registry that's only populated when a session is "bridged" at launch — historical CLI-only sessions (including ones that already have a bridge-session record present in their transcript, e.g. containing a cse_... ID that isn't resolvable via session lookup) don't appear there and can't be resumed from within the app.

Requested: a way to import/attach an existing local session (by directory or session ID) into the harness's tracked session list, so users who've used both the terminal CLI and the hosted app can see and resume all their work from one place, without needing to know or remember which interface a given session originally started in.