fix(server): keep daemon alive when JSONL RPC stdin hits EPIPE

[Elliotwu-7]

Linked issue

Closes #4047

Related: #550 (same crash class, WebSocket-only fix)

Type of change

• Bug fix
• New feature
• Enhancement
• Refactor
• Docs

Reasoning

A dead Claude/OMP JSONL child currently kills the whole daemon. JsonlRpcProcess.send() writes to child.stdin with no error listener and no try/catch. Node treats that write EPIPE as uncaughtException, the worker exits code 1, and every other agent/session on the host goes with it.

#550 already guarded WebSocket send paths. The JSONL RPC stdin pipe used by Claude and OMP was still uncovered. I hit this on desktop-managed 0.7.0-beta.2: two fatal write EPIPE frames from JsonlRpcProcess.send, then Claude runtime exited unexpectedly with code 143, then Worker exited (code 1).

This PR keeps the failure on that transport. The agent turn fails; the daemon stays up.

Goals

• stdin.write() throwing EPIPE does not become process-level uncaughtException
• a stdin 'error' event does not become process-level uncaughtException
• a non-writable stdin fails the in-flight request immediately instead of hanging until the RPC timeout
• later requests on that transport reject with JSONL RPC process is closed

Non-goals

• Changing provider cancel/SIGTERM behavior
• Guarding stdout/stderr error events
• Changing how the agent UI presents a failed turn

QA

Production crash (desktop-managed daemon 0.7.0-beta.2, macOS arm64, Node 24.14.0 inside Electron 41.2.0):

Uncaught exception — daemon crashing
Error: write EPIPE
    at afterWriteDispatched (node:internal/stream_base_commons:159:15)
    at Writable.write (node:internal/streams/writable:508:10)
    at JsonlRpcProcess.send (.../jsonl-rpc-process.js:105:26)
    at new Promise (<anonymous>)
Claude runtime exited unexpectedly  code=143
Worker exited (code 1). Supervisor shutting down.

Regression tests added in jsonl-rpc-process.test.ts for the three Node paths: sync write() throw, stdin 'error' event, and writable === false.

$ npx vitest run packages/server/src/server/agent/providers/jsonl-rpc-process.test.ts --bail=1 --reporter=verbose
 Test Files  1 passed (1)
      Tests  13 passed (13)
   Duration  774ms

Hook on commit: lint, format, and workspace typecheck passed.

Platform	Tested	Notes
iOS	no	daemon-only change
Android	no	daemon-only change
Web	no	daemon-only change
Desktop macOS	yes	unit tests + production stack from local daemon.log
Desktop Windows	no	same Node stdin APIs; not run here
Desktop Linux	no	same Node stdin APIs; not run here

I did not reproduce the crash against a live Claude CLI in this checkout. The two in-memory cases are the Node failure modes from the production stack (write() throw at the send() call site, and an 'error' event with no listener).

Checklist

• One focused change
• npm run typecheck passes
• npm run lint passes
• npm run format passes
• QA evidence
• Tests added or updated where it made sense