[josbeir]

Prepare Synapse for CakePHP 6 and PHP 8.5 on the published cake6 branch. This PR targets cake6, preserving CakePHP 5 support on main.

• Require CakePHP 6, PHPUnit 13, and CakePHP 6 coding standards; allow development dependencies while preferring stable releases. Chronos remains transitive.
• Migrate commands to CakePHP 6's argument and I/O properties, adopt ConsoleIoInterface, and move dependency injection to Cake\Container.
• Update fluent builder return types and test application/fixture setup for CakePHP 6.
• Index the official 6.x documentation by default and target CakePHP 6 / PHP 8.5 in prompts and installation examples.
• Run CI on PHP 8.5 for Linux and Windows, plus a lowest-dependency Linux job.

Validation: latest and lowest resolved dependencies pass 699 tests / 2,377 assertions. PHPStan, PHPCS, Rector, Composer validation, and the CI lowest-version validator pass. Added real CLI MCP initialization/tool invocation tests across cache options, exercised the CakePHP 6 container through MCP discovery/invocation, and added configuration/fallback regression tests. A separate application installs the package successfully using the documented stability settings.

The lowest-dependency run emits vendor deprecation notices. Its validator reports expected warnings for the CakePHP development branch and the available MCP SDK minimum. The latest dependency run has no test issues. Coverage remains approximately 83.1% with PHPUnit 13.

Used the local CakePHP upgrade tool's cakephp60 rules and reviewed/refined its command transformations. Validation used CakePHP 6.x-dev at 3e13a73.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.75949% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.20%. Comparing base (0b84a77) to head (43cac0b).
⚠️ Report is 1 commits behind head on cake6.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/Command/SearchDocsCommand.php	48.14%	14 Missing ⚠️
src/Command/ServerCommand.php	0.00%	7 Missing ⚠️
src/Command/TinkerEvalCommand.php	0.00%	6 Missing ⚠️
src/Command/IndexDocsCommand.php	86.66%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              cake6       #9      +/-   ##
============================================
- Coverage     84.24%   84.20%   -0.05%     
  Complexity      741      741              
============================================
  Files            36       36              
  Lines          2787     2786       -1     
============================================
- Hits           2348     2346       -2     
- Misses          439      440       +1     

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