feat(web): split context injection into producer-declared forms

Injected context reached the transcript as one anonymous shape whose
expanded body was the whole message serialized as inline JSON, so the
escaping collapsed the only readable part — the model-facing prose —
into a single line.

`MessageSource` gains an optional producer-declared `form`, a small
semantic vocabulary of information shapes independent of `kind`: kind
says who produced the context, form says what shape it is, so several
producers may share one presentation. Two values ship.

`instructions` (workspace-context) lists the reconciled files above
their text and keeps the `<system-reminder>` framing verbatim, because
the framing is part of what the model read. `catalog` moves dsh-tool-skill
off the shared plugin kind onto a `skill-catalog` source carrying the
published name/description entries, and the body lists those instead of
re-parsing `<available_skills>` out of the prose. Catalog identity moves
with it: the republish digest now covers the durable entries, deleting
the text-slicing that recovered them from a logged message.

Everything else renders the opaque body — the model-facing text with its
real line breaks, then the remaining provenance as fields. That is the
documented default, not a leftover: a resumed, forked, or foreign log
must render whether or not its producer is mounted here, which is why
the classification lives in the durable source rather than a client-side
table keyed by producer.

packages/cordis/tool-cordis/src/api-catalog.ts

@@ -1809,6 +1809,14 @@ export const TYPE_API: readonly TypeApiEntry[] = [
     name: 'ContentBlockType',
     declaration: 'export type ContentBlockType = keyof ContentBlockMap;',
   },
+  {
+    name: 'ContextForm',
+    declaration: 'export type ContextForm = \'instructions\' | \'catalog\';',
+  },
+  {
+    name: 'ContextFormed',
+    declaration: 'export interface ContextFormed {\n    readonly form?: ContextForm;\n}',
+  },
   {
     name: 'ContinuableCreateRequest',
     declaration: 'export interface ContinuableCreateRequest {\n    readonly sessionId: SessionId;\n    readonly parent: Agent;\n    readonly signal: AbortSignal;\n}',
@@ -2135,7 +2143,7 @@ export const TYPE_API: readonly TypeApiEntry[] = [
   },
   {
     name: 'MessageSourceMap',
-    declaration: 'export interface MessageSourceMap {\n    user: {\n        kind: \'user\';\n    };\n    plugin: {\n        kind: \'plugin\';\n        plugin: string;\n    };\n    model: ModelMessageSource;\n    tool: ToolMessageSource;\n}',
+    declaration: 'export interface MessageSourceMap {\n    user: {\n        kind: \'user\';\n    };\n    plugin: {\n        kind: \'plugin\';\n        plugin: string;\n    } & ContextFormed;\n    model: ModelMessageSource;\n    tool: ToolMessageSource;\n}',
   },
   {
     name: 'ModelMessageSource',