docs: document Plan Mode and resume sandbox limitations

- Add Plan Mode /tmp write limitation to SKILL.md (Step 4) and README
- Document that `codex exec resume` inherits sandbox from original session
- Remove none/low reasoning effort options (minimum is now medium)
- Add .claude to .gitignore (plan files from testing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1 +1,2 @@
-.codex/
+.codex
+.claude