feat: Improve logging

[veracioux]

Changes

• TUI Disruption Prevention: Logs were previously written directly to stderr, potentially corrupting the TUI display. Background mode buffers non-critical logs during TUI operation and flushes them afterward.
• Log Importance: The opt() method allows marking logs as "important" (always visible, even without --print-logs), so users can discover issues more easily
  • The default importance behavior when not explicitly specified is to treat ERROR messages as important, i.e. logged to stderr even without --print-logs
  • State disposal warnings are marked important, so users understand why OC is hanging, and to reduce the steps needed to guide the user while debugging
  • Session.prompt errors are marked unimportant to avoid clutter (i.e. printed only with --print-logs)

Example broken TUI due to --print-logs (before change)

Example messages with the improved logging

(in order for the below example to actually be implemented, .opt({ important: true }) will need to be applied to those logs after #3481 is merged)

$ opencode run hi
Hi!
WARN 2025-10-20T20:38:54 +1000ms service=default waiting for state disposal to complete... (this is usually a saving operation or subprocess shutdown)
WARN 2025-10-20T20:39:03 +8999ms service=default state disposal is taking an unusually long time - if it does not complete in a reasonable time, please report this as a bug