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/tests/workspace-context.spec.ts

@@ -219,6 +219,7 @@ function workspaceChangeContext(scope: string, digest: string): UserMessage {
     content: [{ type: 'text', text: `instructions for ${scope}` }],
     source: {
       kind: 'workspace-instructions',
+      form: 'instructions',
       changes: [{ action: 'set', scope, path: `${scope}/AGENTS.md`, digest }],
     },
   })
@@ -974,6 +975,7 @@ describe('workspace context request injection', () => {
           role: 'user',
           source: {
             kind: 'workspace-instructions',
+            form: 'instructions',
             baseline: true,
             changes: [{ action: 'set', scope: sk('.', 'AGENTS.md'), path: 'AGENTS.md' }],
           },
@@ -1951,6 +1953,7 @@ describe('dynamic nested workspace context injection', () => {
       expect(workspaceContextOf(result)?.source).toMatchObject({ kind: 'workspace-instructions' })
       expect(workspaceContextOf(result)?.source).toMatchObject({
         kind: 'workspace-instructions',
+        form: 'instructions',
         changes: [{
           action: 'set',
           scope: sk('pkg', 'AGENTS.md'),
@@ -2262,6 +2265,7 @@ describe('dynamic nested workspace context injection', () => {
 
       expect(workspaceContextOf(changed)?.source).toMatchObject({
         kind: 'workspace-instructions',
+        form: 'instructions',
         changes: [{ action: 'replace', scope: sk('pkg', 'AGENTS.md'), path: join('pkg', 'AGENTS.md') }],
       })
       expect(blocksText(workspaceContextOf(changed)?.content)).toBe([
@@ -2487,6 +2491,7 @@ describe('dynamic nested workspace context injection', () => {
 
       expect(workspaceContextOf(removed)?.source).toEqual({
         kind: 'workspace-instructions',
+        form: 'instructions',
         changes: [{ action: 'remove', scope: sk('pkg', 'AGENTS.md'), path: join('pkg', 'AGENTS.md') }],
       })
       expect(blocksText(workspaceContextOf(removed)?.content)).toBe([
@@ -2887,6 +2892,7 @@ describe('dynamic nested workspace context injection', () => {
         ],
         source: {
           kind: 'workspace-instructions',
+          form: 'instructions',
           changes: [
             null,
             { action: 'unknown', scope: 'pkg', path: join('pkg', 'AGENTS.md') },
@@ -3073,6 +3079,7 @@ describe('dynamic nested workspace context injection', () => {
       expect(workspaceContextOf(result)?.source).toMatchObject({ kind: 'workspace-instructions' })
       expect(workspaceContextOf(result)?.source).toMatchObject({
         kind: 'workspace-instructions',
+        form: 'instructions',
         changes: [{ action: 'set', scope: sk('pkg', 'AGENTS.md'), path: join('pkg', 'AGENTS.md') }],
       })
       expect(blocksText(workspaceContextOf(result)?.content)).toContain('nested package rule')
@@ -3505,7 +3512,7 @@ describe('workspace context pending state', () => {
     // must not mint per-session pending state.
     expect(commitPendingInstructionContexts(agent, [createUserMessage({
       content: [],
-      source: { kind: 'workspace-instructions', changes: [] },
+      source: { kind: 'workspace-instructions', form: 'instructions', changes: [] },
     })], pending)).toEqual([])
     expect(pending.has(agent.session)).toBe(false)