Add Flymake/Flycheck diagnostic aggregation commands

[morinim]

Summary

Add a command that collects current buffer diagnostics from Flymake or Flycheck and sends them to Codex as a structured batch for root-cause analysis.

The existing my-codex-explain-region-as-error command works on manually selected text. A diagnostics-aware command would let Codex inspect the actual warnings/errors reported by Emacs and identify common causes, cascades, and the smallest likely fix.

Proposed commands

• my-codex-explain-buffer-diagnostics
• Optional later: my-codex-explain-project-diagnostics

Behavior

The buffer-level command should:

• Prefer Flymake when flymake-mode is active.
• Fall back to Flycheck when flycheck-mode is active.
• Include file path, line, column, severity, backend/checker, and message.
• Sort diagnostics by line and column.
• Cap the number of diagnostics, for example at 100, and mention truncation.
• Ask Codex to analyze diagnostics as a batch instead of one-by-one.

Prompt intent

The prompt should ask Codex to identify:

• common root causes
• cascade errors
• missing imports/includes/types/configuration
• the smallest fix likely to remove the largest number of diagnostics

It should discourage proposing one fix per diagnostic unless the diagnostics are genuinely unrelated.

Notes

A project-wide command may be useful later, but it should be scoped carefully. Flymake and Flycheck generally expose diagnostics for buffers that are open and have already been checked, so a project command would likely aggregate visible project buffers rather than represent full project health.