chore: comment cleanup pass ahead of open-sourcing

Removes leftover debug scaffolding (stray Console.WriteLine/NSLog traces,
dead nick_buf_ptr, a no-op --print-config flag now implemented for real),
fixes stale/misleading comments (channel passwords are no longer a "future
M5+" feature, a wrong cross-reference, a stale TlsContext::close() mention,
an incomplete BanRecord::subject_type doc, and a smoke test pointing at a
build/m1-dev preset that no longer exists), strips internal M1-M5 milestone
jargon from comments now that the roadmap is done, trims comments that just
restated the following line, and consolidates a few "why" explanations that
were duplicated 2-3 times in the same file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -36,7 +36,7 @@
 .DS_Store
 Thumbs.db
 
-# Apple / Windows client build artifacts (added in M4)
+# Apple / Windows client build artifacts
 clients/apple/**/build/
 clients/apple/**/*.xcodeproj/xcuserdata/
 clients/apple/**/*.xcodeproj/project.xcworkspace/