feat: expose native subagents and async tasks (#1017)

## Summary

- negotiate native ACP subagent sessions and AIR async task support
- map Claude Agent/Task lifecycle to independent child session streams
- publish background workflow, monitor, MCP, and backgrounded shell work
as async tasks
- attach background shell tasks to their originating tool calls so
clients can update one command card and open its output log
- stream task progress and preserve running, paused, completed, failed,
and stopped states
- keep synchronous shell work, generic tools, and task notifications out
of user message chunks
- preserve typed session failures and root permission/elicitation
requests
- preserve the legacy generic Agent/Task representation and flattened
transcript opt-ins for clients without native subagent capability

## Compatibility

The published ACP SDK does not yet contain PR #1992 types, so the change
isolates the temporary structural cast in `src/acp-subagents.ts`.
Clients that do not negotiate native subagent sessions retain the
existing generic tool-call fallback, including the historical
`_meta["subagent-transcript"]` capability and `forwardSubagentText`
session option. Async tasks use the negotiated `asyncTasks` AIR
capability and remain invisible to clients that do not advertise it.
Claude does not expose a safe targeted child-cancel primitive, therefore
child `cancel` and `close` capabilities remain disabled.

## Validation

- `npm run check`
- `npm run build`
- `env -u ANTHROPIC_BASE_URL npm run test:run` (830 passed, 20 skipped)
- `npm run test:run -- src/tests/async-tasks.test.ts` (5 passed)

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -22,16 +22,16 @@ This tool implements an ACP agent by using the official [Claude Agent SDK](https
 
 Learn more about the [Agent Client Protocol](https://agentclientprotocol.com/).
 
-### Nested subagent transcripts
+### Subagent sessions
 
-ACP 1.2 has no standard subagent tool kind or nested-message relationship. Clients that can render
-nested transcripts can opt in with `clientCapabilities._meta["subagent-transcript"] = true`.
-The agent then forwards subagent text, thinking, and tool calls, relating nested updates to the
-launching Agent/Task call through `_meta.claudeCode.parentToolUseId`. Agent/Task calls are marked
-with `_meta.claudeCode.subagent = true`.
-
-Clients that do not advertise the capability retain the legacy flattened behavior. In both modes,
-the normal Agent/Task tool result is preserved as the protocol-compatible fallback.
+Subagents are exposed only after bilateral capability negotiation. Until the released ACP SDKs
+preserve the draft `clientCapabilities.subagents` field, a supporting client may advertise
+`nativeSubagentSessions` in `_meta.jetbrains.air.capabilities`; the adapter mirrors the capability
+in its initialize response. The canonical field remains supported and takes precedence once it is
+available. Without either client signal, Agent/Task lifecycle keeps its legacy ordinary ACP
+tool-call representation and child interactions stay on the root session. Clients that use the
+historical `_meta["subagent-transcript"]` capability or `forwardSubagentText` session option retain
+the flattened child transcript behavior.
 
 ## Contribution Policy