Update docs: fix README usage examples, add CI badges, document README-sync convention

[trm42]

What changed

README.md

• CI status badges added under the title (Tests, PHPStan, Code style), pointing at the trm42/CacheDecorator workflow files imported in PR Import CI workflows from spatie/package-skeleton-laravel #6.

• Usage examples fixed to compile and run against the current code. Recent commits (US-007) made the configurable properties on CacheDecorator strictly typed, so the old untyped redeclarations (protected $ttl = 300;, protected $prefix_key = 'reports';, etc.) would now be a fatal error. Each example now:

  • types redeclared properties to match the parent (protected ?string $prefix_key, protected array $excludes, etc.);
  • drops the protected $ttl = 300; line — getConfig() runs in the constructor and overwrites $ttl from config, so it never took effect; replaced with a note that TTL comes from config or setTtl();
  • adds #[\Override] on decoratedClass() overrides (US-006);
  • adds @extends CacheDecorator<...> / @extends RepositoryCacheDecorator<...> PHPDoc for IDE/PHPStan type inference (US-005 generics).

  These now follow the same conventions as the test stubs under src/tests/Stubs/.

CLAUDE.md

• Added a "Keep the README in sync" convention: any src/ change affecting the main classes' public-facing surface must be documented in README.md as part of the same change, since the usage examples are a core part of the package DX.

Also included (from prior work on this branch, not yet in master)

• CI workflows imported from the Spatie package skeleton (run-tests, phpstan, fix-php-code-style-issues, dependabot-auto-merge).
• Test stub directory renamed stubs → Stubs to match the PSR-4 namespace.

Why

The documented examples no longer matched the code and would fatal-error on copy-paste; the badges surface CI status; the CLAUDE.md convention prevents the docs from drifting again.

Verification

• composer check green: 28 tests pass, PHPStan reports no errors, Pint passes.
• Each updated example was mirrored in a scratch file and loaded against the real base classes via the autoloader — no property-type fatals.