Node page: show a runner's pane log

Task 7541 asked the Node page to link to pane logs. No console surface serves them yet: jaahd writes `*.pane.log` per runner window (apps/jaahd/fleet_dispatch.py, MicroVM `pane_log` in apps/jaahd/microvmd.py), but nothing ships them to the control plane. Ask: let an operator read the tail of a runner's pane log from the Node page (and from a task's Activity row for a dead run) without reaching the host — e.g. ship the last N KB of a runner's pane log when its session ends abnormally, store it redacted beside the session, and link it from the Node page and Audit › Sessions row. Out of scope: live streaming of panes; liveness or ingest logic. Done means: - A dead run's pane-log tail is viewable from the console, redacted, with a link from the Node page. - Tests pin the redaction and the size cap. Required checks: ./dc check full

A dead run's pane-log tail is now readable in the console (redacted, capped), linked from Node › Health, Audit › Sessions and the task's Activity. PR #6157 merged via the lane. Full gate was red only on tests outside this diff; details in report. Follow-up draft #7698.