feat(web): add nested subagent conversations

packages/client/ui-subagent/README.md

@@ -2,11 +2,13 @@
 
 English | [中文](README.zh.md)
 
-Subagent reference source, browser half: registers the `@`-trigger `subagent` source into `ctx.slash`. Candidates are zero-RPC — filtered from the root `ctx.sessions.list` snapshot captured at registration (children of the per-call projection's session: `parentId` matches, `running`, `displayTitle` contains the query); picking a candidate lands the literal `@label ` text through the slash pipeline (decision 21 plain-text reference), and the source `codec` projects both faces as `@label` — the model serialization stays the raw label until the `@` consumption feature defines a model representation. The source implements no `matchSpace`/`matchEnter` hooks — subagent references never enter command adjudication and ride ordinary prompts into the default sink.
+Web subagent feature owner: contributes the lazily expandable catalog tree to `conversation.session.header.actions`, the unavailable-parent replacement to the conversation composer chain, and the existing `@` reference source to `ctx.slash`.
 
-A session with no running children is simply candidate-less. This phase ships "menu + reference text" only; what consuming an `@label` means (steering the child, resuming a disposed one) is future business work.
+The header action reads `subagentsByParent` and session summaries through the standard `useSessions` hook. After a non-empty catalog arrives it shows the healthy direct-child count and a compact tree in service order. Each healthy row combines its durable label, `running`/`inactive` activity (rendered as `正在处理`/`已完成`), optional log-backed title, and session-summary activity time; corrupt, unsupported, or unavailable rows remain readable but disabled. Expanding a row lazily opens that child's direct catalog and reports every visible branch to the runtime so membership frames cause a debounced refresh only where the tree is being consumed. Selecting any depth calls `SessionsService.openSubagent()` with the row's exact `{parentSessionId, childSessionId}` address. Component-local state owns tree visibility, expanded branches, and keyboard focus. ArrowRight/ArrowLeft expand and collapse branches; ArrowUp/ArrowDown, Home, End, and Escape navigate or close the tree; closing returns focus to the trigger. Styling uses tokens only.
 
-The `/client` export surface is the plugin body (`apply`/`inject`) only; the source object is internal to the registration effect.
+An addressed child with no exact live parent elects the read-only composer entry and explains the recovery path. A child with a live parent keeps the ordinary input chrome, whose Session routes through `subagent.prompt`; this package never receives host context or calls a model-facing tool. The catalog and composer behavior are specified by the [Web subagent conversations Agent Note](../../../.agents/notes/implemented/feature/2026-07-27-web-subagent-conversations.md).
+
+The `@` source remains deliberately separate and inert. Candidates are zero-RPC running children from `ctx.sessions.list`; picking one inserts literal `@label ` text, and the codec projects `@label`. It has no command-adjudication hooks and does not resolve labels into continuation addresses.
 
 ## Model Experience
 
@@ -14,18 +16,18 @@ The `/client` export surface is the plugin body (`apply`/`inject`) only; the sou
 
 #### What the model sees
 
-A picked candidate lands the literal `@label` (the child session's display title) in the draft; the text reaches the model verbatim inside the ordinary user message (`session.prompt`), with no dedicated content block, prompt section, or host-side resolution. No consumption semantics exist yet: the model sees plain text and interprets it unaided.
+Only the legacy `@` reference source affects model input: a picked candidate reaches the ordinary user message as literal `@label`, without a dedicated block or host-side resolution. Catalog browsing, child navigation, persisted transcript viewing, and human continuation UI add no prompt section; continuation content becomes a normal user-role event through the host subagent adapter.
 
 #### Token effect
 
-Conditional and tiny: only a pick (or hand-typing the same text) adds the label's characters to that one user message. Menu browsing adds zero model tokens (candidates never leave the browser).
+Conditional and append-only: the literal `@label` or a human follow-up adds tokens only to its new user message. Catalog and transcript operations add zero model tokens.
 
 #### KV Cache effect
 
-Append-only: the reference is part of a new user message appended after the reusable history prefix. This package never edits earlier request tokens.
+Append-only. This package never edits earlier request tokens.
 
 ## Known Limitations and Deferred Work
 
-- **`@` consumption semantics are unbuilt** — the reference is inert text; wiring it to steer/message the named child (and whether resuming a disposed child is allowed) awaits its own design decision in the ledger.
-- **Candidates are running children only** — completed or disposed subagents never appear, and the roster is the scoped session's direct children (no grandchildren, no cross-session agents).
-- **Labels are display titles, not stable ids** — two children sharing a display title produce indistinguishable references, and a title change orphans previously inserted text. Acceptable while references are inert; a consumption feature must bind to session ids.
+- **The catalog has coarse liveness only** — it cannot show durable outcome, elapsed time, exact Activation state, or a correct cancel button.
+- **The sidebar still contains child sessions** — complete de-duplication needs a scalable durable classifier that does not hide ordinary forks.
+- **`@` references remain display-title text** — duplicate or renamed labels are ambiguous, so they intentionally do not acquire continuation semantics.