feat(web): name context sources and mark steering in the transcript

Every logged non-user user/message collapsed into one identical
「上下文注入」 row, and mid-turn steering rendered in exactly the bubble a
turn-opening prompt uses, so the transcript could not say what had been
added or which message interrupted a running turn.

TranscriptAdapter and the history fold now attach a provenance view to
each context node, computed by contextProvenance() from the durable
source alone: a role (inject, or recall for a cross-session snapshot)
and a producer name read out of the log — instruction paths for
workspace-instructions, session titles for session-reference, the plugin
id for a plugin source, and the bare kind for any other. No client-side
table of producer names, so a renamed or newly mounted producer stays
identifiable without a client release and a foreign log projects like a
live one. ContextInjectionRow titles itself from the role and shows the
name beside it; MessageItem captions durable and pending steering
bubbles.

The caption reverses one clause of the no-interjection-chrome decision,
which removed it because the composer could not steer; composer steering
shipped afterwards without amending that note, so this change supplies
the product decision its reintroduction clause required and corrects the
stale facts left in it.

Fixes #1291

packages/client/runtime/tests/transcript-adapter.spec.ts

@@ -208,7 +208,9 @@ describe('TranscriptAdapter', () => {
           source: { kind: 'plugin', plugin: 'compact' },
         }) }),
       ])
-      expect(adapter.nodes()).toMatchObject([{ kind: 'context', seq: 0 }])
+      expect(adapter.nodes()).toMatchObject([{
+        kind: 'context', seq: 0, provenance: { role: 'inject', label: 'compact' },
+      }])
     })
 
     it('ignores a foreign plugin s replacement user/message', () => {