feat(agent-presets): give a preset a name and a description

A picker showed directory names, so the settings page could only ever list
`standard` / `core-web` / `cordis` and hope the reader knew what they meant.
A preset may now publish display text in an optional `preset.yml` beside
its composition, and the section renders cards — name, description, and the
one in use — instead of rows.

The file carries display text ONLY. `id` is the directory name and `trust`
comes from the root a preset was discovered under, so neither is writable
there: otherwise a locally authored preset could name itself into the
shipped set. It is a separate file because a composition is a top-level list
of plugin rows — YAML cannot carry sibling keys beside it, and a fake
metadata row would hand the Loader something to load.

Every read failure degrades to no metadata; absent, malformed, wrongly
typed, and blank all mean the same thing and the picker falls back to the
id. Presentation is not capability: a preset whose name is broken still
mounts.

The editor gained name and description fields above the YAML, and clearing
both removes the file rather than storing a blank name.

docs/config-catalog.md

@@ -139,7 +139,7 @@ export interface PresetRoot {
 export type PresetTrust = 'system' | 'user'
 ```
 
-Source: [`packages/preset/agent-presets/src/types.ts:29`](../packages/preset/agent-presets/src/types.ts)
+Source: [`packages/preset/agent-presets/src/types.ts:33`](../packages/preset/agent-presets/src/types.ts)
 
 ## `@deepseek-ai/dsh-agent-spine-demo`