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>

.claude/settings.json

@@ -0,0 +1,20 @@
+{
+  "permissions": {
+    "allow": [
+      "Read(**)",
+      "Write(examples/**)",
+      "Write(output/**)",
+      "Bash(python3.12:*)",
+      "Bash(python3:*)",
+      "Bash(.venv/bin/python:*)",
+      "Bash(make:*)",
+      "Bash(git:*)",
+      "Bash(pip:*)",
+      "Bash(pip3:*)",
+      "Bash(pip3.12:*)",
+      "Bash(ls:*)",
+      "Bash(which:*)",
+      "Bash(find:*)"
+    ]
+  }
+}