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

apps/web/tests/snapshots/code-mode-round/ui.expected.md

@@ -10,10 +10,10 @@
 - button "Branch into a new conversation" [disabled]:
   - img
 - text: Available only on the last message of a completed turn
-- button "Context injection":
+- button "Context injection · @deepseek-ai/dsh-system-prompt":
   - img
   - img
-  - text: Context injection
+  - text: Context injection · @deepseek-ai/dsh-system-prompt
 - 'button "Think The user wants me to write a single `run_code` program that:"':
   - img
   - img