fix(server): install uncaught error handlers for library consumers

[Alexander-Ollman]

Summary

bin/cli.ts has long installed process.on('uncaughtException') and process.on('unhandledRejection') handlers that log and swallow socket-level errors (EPIPE, ECONNRESET) and SDK subprocess crashes instead of letting them kill the proxy. Until this PR those handlers lived only in the CLI, so library consumers calling startProxyServer in-process had no safety net.

In our setup (era-code starts meridian via await import('@rynfar/meridian') and startProxyServer({...}) so it can run inside the same Node process as the OpenCode wrapper) a single mid-stream EPIPE was crashing the host process and orphaning OpenCode, which then saw ECONNRESET on the in-flight request, then ConnectionRefused on retries, and entered an unbounded retry loop with Cannot connect to API: Unable to connect… until the user killed it.

This PR moves the safety net into the library entry behind an opt-in flag, so any library consumer can enable it (and the bundled CLI already does).

Changes

• src/proxy/types.ts — add installProcessErrorHandlers?: boolean to ProxyConfig. Defaults to false to preserve any handlers a library consumer has already installed.
• src/proxy/server.ts — export installProxyProcessErrorHandlers() (idempotent). startProxyServer calls it when installProcessErrorHandlers: true is passed.
• bin/cli.ts — passes installProcessErrorHandlers: true into start({...}) and removes the now-duplicate handler block. The CLI's behavior is byte-identical to before; the handlers are now installed in exactly one place.
• src/__tests__/install-process-error-handlers.test.ts — verifies the helper attaches both listeners and is idempotent on repeat calls.

Test plan

• bun test src/__tests__/install-process-error-handlers.test.ts — 2 pass
• bun build bin/cli.ts src/proxy/server.ts --outdir dist --target node --splitting --external @anthropic-ai/claude-agent-sdk --entry-naming '[name].js' succeeds; both dist/cli.js and dist/server.js parse cleanly with node --check
• Manual: confirmed installProxyProcessErrorHandlers symbol present in the built dist/server.js bundle
• Maintainer: full bun test to confirm no regressions in adjacent suites

Compatibility

• Existing CLI users: identical behavior — handlers still installed, same log format ([PROXY] Uncaught exception (recovered): …).
• Existing library consumers: identical behavior unless they explicitly pass installProcessErrorHandlers: true (or call the new exported helper). No silent change to global process state.