File the concrete `/session-continuity:doctor` retrofit once `4a9d` is decided

[talgolan]

Its five report rows are deterministic like backlog/learnings/help/update,
but it reads live installation state (hook registration, plugin cache path,
file staleness) rather than a static repo file, so it needs a different
renderer shape than hooks/lib/render.sh's file-parsing model — Phase 1 of
the determinism program deliberately did not touch it. Distinct from 4a9d:
that item is the architectural decision (should /doctor become a zero-turn
script at all); this item is the concrete retrofit work once that decision
lands.

[talgolan]

4a9d already decided (shared status stays narrow; doctor stays separate). Concrete retrofit filed as #68 — zero-turn via doctor-report.sh, not render.sh's file model.