fix(agent-presets): bound the mount registry on a host that never reads it

Records are pruned by observation rather than by a disposal hook, for the
reason the module already states: three different owners can tear a
subtree down, and a cleared `uid` is what they share. That leaves the
pruning to whoever reads — and the only production reader is the
invariant companion, whose package is a development composition a
shipped host never loads.

So a live host pruned nothing: every session ever composed left a record
retaining its whole disposed subtree, since the fiber holds its config
and that config is the key its EntryTree is stored under.

Prune on the mount path too. Every session takes it, which bounds the
set at one generation of dead records instead of one per session.

packages/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/README.md
-README.md: 365659617c97c44dd0f30fbcd3347b6438024eb3
-README.zh.md: 9edabd67ea728e77e2863a32c250675a5b9359f8
+README.md: b736aa5dc9d0e9313d652d40c3f4834456dccbb4
+README.zh.md: 53081b5e8c2d465dc866644eae78bebf0c4fc3a1