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.

docs/module-graph.md

@@ -220,6 +220,9 @@ flowchart TD
   subgraph group_mcp["packages/mcp"]
     pkg_mcp_client["mcp-client"]
   end
+  subgraph group_preset["packages/preset"]
+    pkg_agent_presets["agent-presets"]
+  end
   subgraph group_pty["packages/pty"]
     pkg_pty["pty"]
     pkg_pty_local["pty-local"]
@@ -335,6 +338,9 @@ flowchart TD
   pkg_credentials --> pkg_invariants
   pkg_frontend_static --> pkg_host_webserver
   pkg_frontend_static --> pkg_invariants
+  pkg_agent_presets --> pkg_invariants
+  pkg_agent_presets --> pkg_paths
+  pkg_agent_presets --> pkg_scope
   pkg_helper --> pkg_brand
   pkg_helper --> pkg_invariants
   pkg_helper --> pkg_subprocess
@@ -1132,6 +1138,7 @@ flowchart TD
 | [`client-ui-trajectory`](../packages/client/ui-trajectory) | `client` | [`client-runtime`](../packages/client/runtime), [`client-ui-primitives`](../packages/client/ui-primitives), [`invariants`](../packages/support/invariants) |
 | [`credentials`](../packages/credentials/credentials) | `credentials` | [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants) |
 | [`frontend-static`](../packages/host/frontend-static) | `host` | [`host-webserver`](../packages/host/webserver), [`invariants`](../packages/support/invariants) |
+| [`agent-presets`](../packages/preset/agent-presets) | `preset` | [`invariants`](../packages/support/invariants), [`paths`](../packages/util/paths), [`scope`](../packages/core/scope) |
 | [`helper`](../packages/sdk/helper) | `sdk` | [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`subprocess`](../packages/subprocess/subprocess) |
 | [`telemetry`](../packages/sdk/telemetry) | `sdk` | [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`paths`](../packages/util/paths) |
 | [`settings`](../packages/settings/settings) | `settings` | [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants) |