fix(daemon): add backpressure control and command serialization to prevent IPC EAGAIN

[shohu]

Summary

Fixes the daemon-side root causes of EAGAIN (os error 35 on macOS / error 11 on Linux) that crash the Rust CLI during IPC reads.

This complements #329 (CLI-side retry logic) by preventing the conditions that trigger EAGAIN in the first place.

Changes

src/browser.ts — Configurable Playwright timeout via environment variable

• Add getDefaultTimeout() helper that reads AGENT_BROWSER_DEFAULT_TIMEOUT env var
• Replace all hardcoded setDefaultTimeout(60000) calls (5 locations) with setDefaultTimeout(getDefaultTimeout())
• CDP and recording contexts (10s timeout) are not affected
• Default remains 60s when env var is unset — fully backward compatible

src/daemon.ts — Backpressure-aware writes + command serialization

• Add safeWrite() helper that waits for drain event when socket.write() returns false, with proper cleanup on close/error
• Serialize command execution per socket via a queue to prevent concurrent socket.write() calls that cause kernel buffer contention
• Add .catch() guards for unhandled rejection safety

Root Cause Analysis

Two daemon-side issues combine to cause EAGAIN:

1. Playwright timeout > CLI IPC timeout: setDefaultTimeout(60000) means Playwright can block for up to 60s, but the Rust CLI times out earlier. The daemon never responds, and the CLI's read_line() hits EAGAIN.

2. Uncontrolled socket.write() concurrency: Multiple async command handlers can call socket.write() in parallel. When payloads are large (e.g., snapshot responses), the kernel buffer fills up and subsequent writes/reads fail with EAGAIN.

Testing

• All 363 existing tests pass (9 skipped)
• Prettier formatting verified
• TypeScript compilation clean
• Manually tested with a heavy React Native/Expo app (~1000+ DOM nodes): 10 consecutive snapshot commands complete without os error 35

Usage

# Set a shorter timeout (e.g., 30s) to prevent daemon hangs
export AGENT_BROWSER_DEFAULT_TIMEOUT=30000

Refs #322

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[ctate]

This is so good - thanks @shohu