feat(tui): add message_limit config and fix export truncation

[n24q02m]

Issue for this PR

Closes #9930
Closes #14948

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

1. Fix TUI lag and 100% CPU on long sessions (Add configurable scrollback/message limit to reduce TUI lag #9930, High CPU usage (100%+) during LLM streaming in long sessions due to O(n) text buffer rendering #6172): Introduces the tui.message_limit config option. By setting this to an integer (e.g. 50), OpenTUI will only render the last N messages of a session. This completely avoids the O(n) layout calculation loops that previously crashed the app when scrolling long histories. The default remains "none" to preserve backwards compatibility.
2. Fix JSON truncation on opencode export via piped stdout ([BUG] opencode export outputs truncated JSON when piped #14948, Bug: Intermittent truncated stdout when piping opencode export #2803): When users piped the output of opencode export to tools like jq, the main process often exited via process.exit() before NodeJS could fully flush the asynchronous stdout stream. This PR fixes it by properly awaiting a zero-length write callback in the export handler and introducing a setImmediate() microtask loop before the final app teardown.

How did you verify your code works?

Built and compiled locally.

1. Export Test: Generated a massive session and piped opencode export <session> | jq . -> It successfully parsed the full 15MB file without premature truncation errors.
2. TUI Test: Tested tui: {message_limit: 5} in opencode.json, opened a long session and observed that only the last 5 messages were rendered, with CPU usage dropping to near 0%.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[n24q02m]

Rebased onto latest dev to resolve conflicts. Also removed the stale Closes #2803 reference (that issue was already fixed by #3049 in Oct 2025). The +13K lines addition is from the auto-generated openapi.json spec.

[n24q02m]

Closing — I'm no longer actively using OpenCode. Feel free to cherry-pick any changes if useful.