feat: custom check plugins — extensible validation pipeline

[hermanngeorge15]

Description

Allow users to register custom check functions that run as part of the validation pipeline alongside the built-in checks.

Acceptance Criteria

• Plugin interface: { name: string; run(specContent: string, changedFiles: string[]): CheckResult }
• New input custom-checks pointing to a JS/TS file exporting check functions
• Custom checks appear in PR comment alongside built-in checks
• Documentation with example custom check

Milestone

v0.1.0