Follow-up to the salvaged success-path removal: installs that already
repaired (or predate the cleanup) still carry leaked ~1 GB parked venvs.
When the runtime probes safe, reclaim aged (>1h) stale markers next to
the live venv — age-gated to avoid racing an in-flight sibling repair,
boundary-checked via _remove_tree so symlinked names can't escape the
checkout. Also drop the now-stale 'before removing the parked venv'
user guidance in update_cmd.
Tests: success-path removal, safe-path sweep (aged removed, fresh kept).
The rebase onto main (which landed 3a69e34702 touching the two functions
this branch moves to update_cmd.py) resolved main.py to the moved-out
state; this commit re-applies the perf commit's function bodies at their
new home so no behavior from main is lost. Bodies extracted verbatim
from origin/main via AST.
