feat(dagger): Allow to pass GitHub/GitLab env variable for PR detection

[javirln]

Add support for automatic pull request and merge request metadata collection when Chainloop runs via Dagger inside Github Actions or Gitlab CI pipelines.

Changes

Dagger Module (extras/dagger/)

• Extended Init() function with optional parameters to accept parent CI context (Github and Gitlab environment variables)
• Added internal ParentCIContext struct to organize parent CI environment data
• Updated cliContainer() to inject parent CI environment variables and mount Github event file into the container

PR Detection (pkg/attestation/crafter/prmetadata.go)

• Added support for DAGGER_PIPELINE runner type in DetectPRContext()
• Detection logic checks for Github context first, then Gitlab context when running in Dagger

Documentation (extras/dagger/README.md)

• Added "PR/MR Auto-Detection in Parent CI" section with examples for both Dagger SDK (Go) and Dagger CLI usage
• Documented all required environment variables for Github Actions and Gitlab CI

All parameters are optional and backward compatible. PR detection fails gracefully if environment variables are missing or invalid.

[jiparis]

Looks good to me!