fix(agent-presets): let the invariant follow the roster it actually scans

The service moved its "is a roster composed" reads to the derived root set;
the invariant companion still read `config.roots`. In the shape this change
exists for — an app configures nothing and the roster is the harness home
alone — that made the advisory warning fire while the fail-loud invariant
stayed silent, so an agent could address a model against an empty global
layer unchecked.

Both now read one source: `roots` exposes the resolved set, and the invariant
asks it. That decides the behavior deliberately rather than by omission — a
composition that mounts the roster now fails an unjoined agent whether its
roots were configured or derived, and `includeUserRoot: false` with no
configured roots is how a deployment keeps its agents on the host plane. Both
shapes are pinned; the derived-only case fails against the old predicate.

Three pieces of prose went stale with the first commit: the web-app bundle
comment still called the writable root an assembly fact patched in by
AppCLIEntry (removed in the profile-plugin-bundles refactor — `composeProfile`
owns it now, and only for the shipped root), and the shipped skill and its
Agent Note still called both roots "configuration". The README gains the
resolved-roster reader and the discoverable-but-undeletable preset a second
writable root produces.

apps/cli/config/agent-presets/cordis/skills/editing-cordis-compositions/SKILL.md

@@ -25,13 +25,13 @@ Two planes, and the choice is not about how "agent-related" something feels —
 
 A preset is a directory holding one `agent.cordis.yml`, optionally beside a `preset.yml` carrying display metadata — `name` and `description` (and, for shipped presets, a roster `order`). Write the metadata too: a preset without it shows up in every picker as its bare directory name.
 
-Locally authored presets live one directory per preset under `${DSH_HOME:-$HOME/.dsh}/.agent-presets/`, and the shipped set sits beside the deployment's own config. Use those when the user asks where to look. Both roots are configuration rather than fixed locations, though, and no call reports them — `authorable` says only whether a writable one exists — so take the path you actually read or edit from `list()` or `resolve()`, which is also where `copy()` reports what it just created.
+Locally authored presets live one directory per preset under `${DSH_HOME:-$HOME/.dsh}/.agent-presets/`, and the shipped set sits beside the deployment's own config. Use those when the user asks where to look. A deployment can configure other roots, so the path you read or edit comes from `list()` or `resolve()` — which is also where `copy()` reports what it just created.
 
 ## The roster service
 
 `ctx.agentPresets` owns discovery, authoring, and mounting. You reach it by mounting a temporary plugin that injects it and registers a tool for yourself — `cordis_mount` returns only the mount acknowledgement, so a registered tool is how a service answer gets back to you, and it becomes callable on your next step.
 
-Read `cordis_inspect what:"api" name:"agentPresets"` for the current signatures before writing the code. The four calls this skill relies on:
+Read `cordis_inspect what:"api" name:"agentPresets"` for the current signatures before writing the code. What this skill relies on:
 
 - `list()` — every preset with its `id`, `trust` (`system` for the shipped set, `user` for authored ones), and the absolute `path` of its composition file. This is how you locate any composition without knowing the install layout; the directory is that path's parent.
 - `read(id)` — one preset's composition text, without a file tool or a path.