fix(memory): Fix timeout, interval, and subscription cleanup

[sauerdaniel]

Summary

Fix various small memory leaks from uncleaned timeouts, intervals, and event subscriptions across the codebase.

Fixes #9156

Problems

1. WebFetch Timeout (webfetch.ts)

When fetch throws an error, the timeout is not cleared.

2. Models setInterval (models.ts)

A setInterval is created but never cleared, even when the module is unloaded.

3. Bus.subscribe Return Value (github.ts)

The unsubscribe function returned by Bus.subscribe() is not captured or called.

Solution

WebFetch - Use try/finally

const timeout = setTimeout(...)
try {
  const response = await fetch(...)
  return response
} finally {
  clearTimeout(timeout)
}

Models - Track interval for cleanup

const intervalId = setInterval(...)
process.on("exit", () => clearInterval(intervalId))

Bus.subscribe - Capture unsubscribe

Store the unsubscribe function for later cleanup.

Changes

• packages/opencode/src/util/webfetch.ts - Clear timeout in finally block
• packages/opencode/src/provider/models.ts - Add interval cleanup on exit
• packages/opencode/src/github/github.ts - Capture Bus.subscribe return value

Testing

• [x] TypeScript compilation passes (bun turbo typecheck)
• [x] Unit tests pass (725 tests, 0 failures)

Note: Manual testing of timeout/interval cleanup requires runtime verification which was not performed.