feat: add Cursor, Codex, and OpenCode harnesses — maximize every AI coding tool

- AGENTS.md: universal cross-tool file read by Claude Code, Cursor, Codex, and OpenCode
- .cursor/: 15 hook events via hooks.json, 16 hook scripts with DRY adapter pattern,
  29 rules (9 common + 20 language-specific) with Cursor YAML frontmatter
- .codex/: reference config.toml, Codex-specific AGENTS.md supplement,
  10 skills ported to .agents/skills/ with openai.yaml metadata
- .opencode/: 3 new tools (format-code, lint-check, git-summary), 3 new hooks
  (shell.env, experimental.session.compacting, permission.ask), expanded instructions,
  version bumped to 1.6.0
- README: fixed Cursor section, added Codex section, added cross-tool parity table
- install.sh: now copies hooks.json + hooks/ for --target cursor

.cursor/rules/python-coding-style.md

@@ -0,0 +1,42 @@
+---
+description: "Python coding style extending common rules"
+globs: ["**/*.py", "**/*.pyi"]
+alwaysApply: false
+---
+# Python Coding Style
+
+> This file extends the common coding style rule with Python specific content.
+
+## Standards
+
+- Follow **PEP 8** conventions
+- Use **type annotations** on all function signatures
+
+## Immutability
+
+Prefer immutable data structures:
+
+```python
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class User:
+    name: str
+    email: str
+
+from typing import NamedTuple
+
+class Point(NamedTuple):
+    x: float
+    y: float
+```
+
+## Formatting
+
+- **black** for code formatting
+- **isort** for import sorting
+- **ruff** for linting
+
+## Reference
+
+See skill: `python-patterns` for comprehensive Python idioms and patterns.