fix(config): respect $EDITOR when config editor is unset

[syf2211]

Summary

When editor is not set in config.yaml, aichat should use $VISUAL or $EDITOR before falling back to nano/notepad.

Motivation

Fixes #1527. After #1082, Config::editor() validated the resolved editor with which::which(). If $EDITOR pointed to a valid editor that which could not resolve (common with custom install paths or editor commands with arguments), resolution failed and REPL editing fell back to nano.

Changes

• Add resolve_editor() with priority: config → VISUAL → EDITOR → platform fallback
• Trust explicit config/env editor values without a which check
• Only validate the nano/notepad fallback with which
• Parse editor commands via shell_words in edit_file and REPL buffer editor
• Add unit tests for editor resolution

Tests

cargo test

All 26 tests pass.

Notes

Blank/whitespace editor: values in config are treated as unset so $EDITOR is honored.