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/host/apiproxy/src/api/agent-presets.schema.ts

@@ -16,6 +16,7 @@ export const agentPresetEntrySchema = z.object({
   isDefault: z.boolean(),
   name: z.string().optional(),
   description: z.string().optional(),
+  broken: z.string().min(1).optional(),
 }) satisfies z.ZodType<Wire<AgentPresetEntry>>
 
 /** agentPreset.list request payload. */

packages/host/apiproxy/src/api/agent-presets.ts

@@ -33,6 +33,13 @@ export interface AgentPresetEntry {
   readonly name?: string
   /** One sentence on what the preset is for, when it published one. */
   readonly description?: string
+  /**
+   * Why this preset cannot compose a session, absent when it can. A broken
+   * preset stays listed — its directory still occupies the id, so a surface
+   * must be able to show and delete it — but offering it for selection would
+   * only defer this reason to a failed session start.
+   */
+  readonly broken?: string
 }
 
 /** agent-preset-domain unary methods (the map key agentPreset.* of RpcMethodMap). */