Add configurable double-Esc interrupt window

[anntnzrb]

Summary

• add session_interrupt_timeout_ms to TUI config (default 5000, bounds 500–10000) and document it
• use the configured timeout for the prompt double-Esc interrupt window instead of the hardcoded 5s
• add config tests for default, in-range acceptance, and out-of-range rejection

Example config:

{
  "$schema": "https://opencode.ai/config.json",
  "tui": {
    "session_interrupt_timeout_ms": 3000
  }
}

Fixes #5785

Testing

• bun test packages/opencode/test/config/config.test.ts (fails: existing test "migrates autoshare to share field" expects "auto" but got "manual")