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/examples/agent-spine-demo/tests/agent-core.spec.ts

@@ -467,9 +467,7 @@ describe('dsh-agent-spine-demo bundle', () => {
       expect(loadedRequest).toContain('Use the freshly loaded body.')
 
       const transcript = handle.agent.session.events.flatMap<Record<string, unknown>>((event) => {
-        if (event.type === 'user/message'
-          && event.data.source.kind === 'plugin'
-          && event.data.source.plugin === 'dsh-tool-skill') {
+        if (event.type === 'user/message' && event.data.source.kind === 'skill-catalog') {
           return [{
             type: event.type,
             source: event.data.source,
@@ -503,8 +501,14 @@ describe('dsh-agent-spine-demo bundle', () => {
           },
           {
             "source": {
-              "kind": "plugin",
-              "plugin": "dsh-tool-skill",
+              "entries": [
+                {
+                  "description": "Hot-added skill",
+                  "name": "hot-skill",
+                },
+              ],
+              "form": "catalog",
+              "kind": "skill-catalog",
             },
             "text": "<system-reminder>
         A skill is a reusable set of task-specific instructions. The following skills are available in this session: