Portable setup: venv, Claude Code + Codex project config

Makes the skill work on any laptop regardless of installed Python version
or LLM harness, without per-command approval prompts or missing deps.

- Makefile: auto-detect Python (venv > 3.12 > 3.11 > 3.10 > python3)
  and new `make venv` target that picks the best Python at creation time
- .claude/settings.json: project-level Claude Code permissions
  (Write to examples/ and output/, common bash commands pre-authorized)
- .codex/config.toml: Codex sandbox config with network_access=true,
  approval_policy=never, sandbox_mode=workspace-write — fixes
  `make venv` failing with "No matching distribution" in Codex
- CLAUDE.md / AGENTS.md: document the `make venv` flow, drop all
  hardcoded python3.12 references in favor of make targets
- CLAUDE.md: add Karpathy-style coding guidelines (think before coding,
  simplicity first, surgical changes, goal-driven execution)
- .gitignore: add .venv/

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

.gitignore

@@ -1,3 +1,4 @@
+.venv/
 __pycache__/
 *.pyc
 *.pyo