fix(agent-presets,web): broken presets are roster rows, not gaps

A hand-damaged preset was silent until the worst moment. An unparsable
composition listed as an ordinary selectable row and failed only at the
next session start — set as default, every new session failed. A
directory whose composition file was deleted vanished from the roster
while still occupying its id: copy answered "delete the existing preset
first" while remove answered "not found", a dead end.

Discovery now owns health: every id-shaped directory is a roster slot,
broken when its composition is missing or unloadable, checked with the
loader's own entryListSchema dialect (!!js included) so health never
rejects what the loader accepts. `broken` rides AgentPreset, the
agentPreset.list entry, and the UI row; mount/recompose/standingKeyFor
refuse broken up front with the discovery-reported reason, while
resolve/read/remove still answer. The section renders marked red cards —
unselectable, uncopyable, deletable, location kept on custom rows — and
both pickers drop broken rows entirely.

The cordis preset's persona now forbids editing the shipped install
(corrupting cordis would disable the mode itself) and points authoring
at $DSH_HOME/.agent-presets; its skill teaches preset.yml metadata, the
copy-first workflow, the one-escalation sandbox reality, and honest
verification. Exercised live: asked to edit the shipped composition the
composed agent refuses citing both rules; asked for real presets (simple
and complex) it lands them under the user root with one approved
escalation each and self-checks with the loader dialect.

packages/preset/agent-presets/src/index.ts

@@ -153,6 +153,10 @@ export class AgentPresets extends Service {
 
   /**
    * Resolve one preset by id.
+   *
+   * A broken preset resolves — deleting one, reading one, and reporting one
+   * all need the row — and the mounting paths refuse it AFTER resolution
+   * through {@link resolveMountable}.
    * @param id - the preset id, or `undefined` for {@link defaultId}.
    * @returns the resolved preset.
    * @throws when no configured root supplies that id.
@@ -167,6 +171,24 @@ export class AgentPresets extends Service {
     return found
   }
 
+  /**
+   * Resolve one preset that is about to compose an agent, refusing a broken
+   * one with its discovery-reported reason. Failing here rather than inside
+   * the loader keeps the answer the same for every unloadable shape — ghost
+   * directory, unparsable YAML, rowless list — and spends no mount attempt
+   * on a composition discovery already read as unusable.
+   * @param id - the preset id, or `undefined` for {@link defaultId}.
+   * @returns the resolved, mountable preset.
+   * @throws when the preset is unknown or discovery reports it broken.
+   */
+  private async resolveMountable(id?: string): Promise<AgentPreset> {
+    const preset = await this.resolve(id)
+    if (preset.broken !== undefined) {
+      throw new PresetMountError(preset.id, preset.broken)
+    }
+    return preset
+  }
+
   /**
    * Standing mounts by preset id, single-flight so two agents racing the
    * first use of one preset share one composition. A settled failure is
@@ -198,7 +220,7 @@ export class AgentPresets extends Service {
     if (agentKey === undefined) {
       throw new Error('agent-presets: refusing to compose an unscoped context; the scope key is what joins an agent to its preset')
     }
-    const preset = await this.resolve(id)
+    const preset = await this.resolveMountable(id)
     const standing = await this.ensureStanding(preset)
     setScopeParent(agentKey, standing.key)
     return preset
@@ -314,7 +336,7 @@ export class AgentPresets extends Service {
     if (agentKey === undefined) {
       throw new Error('agent-presets: refusing to recompose an unscoped context')
     }
-    const preset = await this.resolve(id)
+    const preset = await this.resolveMountable(id)
     const standing = await this.ensureStanding(preset)
     setScopeParent(agentKey, standing.key)
     return preset
@@ -332,7 +354,7 @@ export class AgentPresets extends Service {
    * @throws when the preset is unknown or its composition is unusable.
    */
   async standingKeyFor(id?: string): Promise<ScopeKey> {
-    const preset = await this.resolve(id)
+    const preset = await this.resolveMountable(id)
     return (await this.ensureStanding(preset)).key
   }