fix: live-runs injection — summarize the backlog, flag zombie runs as stale

[aarontrowbridge]

Problem — The live stack-state block lists every run directory individually. On a fleet with history that is a 120-line wall of done (F=…) entries that buries the one signal the onset greeting needs, and it reports any run without a FINISHED marker as "solving" with no age check — so an eight-day-old zombie was presented to the user as "one run still solving".

Approach — Summarize in the plugin's runs builder: live runs (unfinished, younger than 12 h) shown individually with age; older unfinished runs flagged STALE — probably dead, do not present as live with their start date; finished runs collapsed to a single backlog line (count, best F, latest run) with a pointer to the full history in the runs dir.

Acceptance Criteria

• A fresh unfinished run renders as solving (<age> old).
• An unfinished run older than 12 h renders as STALE — no FINISHED since <date> and never as solving.
• Finished runs render as exactly one backlog: line (count, best F, latest), not one line per run.
• No runs → no live-runs section at all.

Source

Follow-up to #306 (observed live: the onset greeting mined a 120-run wall and reported an Aug-14 zombie as "one run still solving").

Notes

Verification: plugin suite 17/17 (3 new live-runs cases), typecheck clean, extension suite 1209 passed / 5 skipped / 100 files.