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/ui-conversation/README.md

@@ -14,7 +14,7 @@ Approvals take over the composer through the chain this package declares: `Appro
 
 The session header declares and renders the session-scoped `'conversation.session.header.actions'` list beside the title, allowing feature plugins to contribute controls without entering the skeleton. The composer chain currency includes the current conversation `session`; ui-subagent selects one-shot or parent-unavailable addressed sessions for reason-specific read-only copy, while the ordinary InputBar keeps every addressed child Send-only because the continuation service exposes no public per-Activation cancellation operation and `session.cancel` would bypass its ownership.
 
-Logged non-user messages render as a default-collapsed `上下文注入` disclosure. It shares the Tool calls header geometry and interaction with `ToolRow` through the package-internal `DisclosureRow`, while retaining context semantics: the expanded body follows its content height up to a 141px scrolling cap, shows inline JSON for both `content` and `source`, and synthesizes no tool state, summary, or keyed toolview dispatch ([decision](../../../.agents/notes/implemented/feature/2026-07-30-web-context-injection-disclosure.md)).
+Logged non-user messages render as a default-collapsed disclosure whose header names the role the runtime projected for the message — `上下文注入` for an injection, `跨会话召回` for a recalled session — followed by the producer name that projection read out of the durable source, so a reader distinguishes a skill catalog from a workspace instruction file or a recalled session without expanding. A source that names no producer shows the role alone. The header shares the Tool calls geometry and interaction with `ToolRow` through the package-internal `DisclosureRow`, while retaining context semantics: the expanded body follows its content height up to a 141px scrolling cap, shows inline JSON for both `content` and `source`, and synthesizes no tool state, summary, or keyed toolview dispatch ([disclosure decision](../../../.agents/notes/implemented/feature/2026-07-30-web-context-injection-disclosure.md), [provenance decision](../../../.agents/notes/implemented/feature/2026-08-04-web-context-source-and-steer-marks.md)). A durable or pending steering bubble carries an `插话` caption above it, the only thing distinguishing a mid-turn interjection from the turn-opening prompt that shares its bubble.
 
 A Think row stays collapsed by default and exposes live reasoning throughput without expanding the chain of thought: while its reasoning block is the streaming tail, the summary switches from the settled first line to the latest non-blank line and its one-line scrollport follows each delta to the inline end. Expanding the row removes the moving summary and leaves the full reasoning in ordinary page flow, so page reading never fights an internal follower; settlement restores the stable first-line summary at the left edge ([decision](../../../.agents/notes/implemented/feature/2026-08-02-web-thinking-tail-scroll.md)).