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/swift-testing.md

@@ -0,0 +1,45 @@
+---
+description: "Swift testing extending common rules"
+globs: ["**/*.swift", "**/Package.swift"]
+alwaysApply: false
+---
+# Swift Testing
+
+> This file extends the common testing rule with Swift specific content.
+
+## Framework
+
+Use **Swift Testing** (`import Testing`) for new tests. Use `@Test` and `#expect`:
+
+```swift
+@Test("User creation validates email")
+func userCreationValidatesEmail() throws {
+    #expect(throws: ValidationError.invalidEmail) {
+        try User(email: "not-an-email")
+    }
+}
+```
+
+## Test Isolation
+
+Each test gets a fresh instance -- set up in `init`, tear down in `deinit`. No shared mutable state between tests.
+
+## Parameterized Tests
+
+```swift
+@Test("Validates formats", arguments: ["json", "xml", "csv"])
+func validatesFormat(format: String) throws {
+    let parser = try Parser(format: format)
+    #expect(parser.isValid)
+}
+```
+
+## Coverage
+
+```bash
+swift test --enable-code-coverage
+```
+
+## Reference
+
+See skill: `swift-protocol-di-testing` for protocol-based dependency injection and mock patterns with Swift Testing.