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/context/workspace-context/src/index.ts

@@ -126,6 +126,7 @@ export function apply(ctx: Context, config: Config): void {
         content: baselineMessage.content,
         source: {
           kind: 'workspace-instructions',
+          form: 'instructions',
           baseline: true,
           changes: [...baseline.changes.values()],
         },

packages/context/workspace-context/src/state.ts

@@ -39,6 +39,8 @@ const FILE_TOUCH_TOOL_NAMES = new Set(['read', 'write', 'edit'])
 /** Durable provenance and reconciliation facts for one workspace context. */
 export interface WorkspaceInstructionSource {
   kind: 'workspace-instructions'
+  /** Every workspace context carries instructions read out of a file (the `instructions` context form). */
+  form: 'instructions'
   /** Marks the complete startup/resume baseline rather than a later delta. */
   baseline?: true
   changes: WorkspaceInstructionChange[]
@@ -87,7 +89,7 @@ export interface ReconciledInstructionContext {
 function workspaceContextHook(text: string, changes: WorkspaceInstructionChange[]): UserMessage {
   return createUserMessage({
     content: [{ type: 'text', text }],
-    source: { kind: 'workspace-instructions', changes },
+    source: { kind: 'workspace-instructions', form: 'instructions', changes },
   })
 }