Pressing `d` on the highlighted row in the resume picker prompts
`delete? y/n`; `y` deletes the session (DB row + on-disk transcript
files), anything else cancels. The active session is excluded from
deletion server-side.
Adds a new `session.delete` JSON-RPC handler that wraps
`SessionDB.delete_session`, forwarding the per-profile `sessions/`
directory so transcripts get cleaned up alongside the row.
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