Report missing session workspaces in doctor

[jongio]

Problem

Dispatch can mark sessions whose working directories are missing in the TUI, but the command line diagnostics do not summarize stale or deleted workspaces. Users who script health checks have to open the UI to see that state.

Proposed solution

Extend dispatch doctor with a workspace summary that counts missing working directories and shows a small sample of affected paths. Include the same data in dispatch doctor --json.

Acceptance criteria

• dispatch doctor reports the count of sessions with missing working directories.
• dispatch doctor --json includes the count and sample paths.
• The check handles unreadable or missing session stores without panics.
• Tests cover zero missing paths, some missing paths, and JSON output.