GitLab provider adapter with cross-adapter parity tests

[gkanitz]

Parent

#1

What to build

The second Provider implementation: a GitLab adapter covering the full ActivitySet (MRs authored/merged, reviews/approvals given, comments written/received, merge timing, rework signals), with identity bound to the immutable GitLab account ID and org-scoping via a group access token with read-API scope. This slice exists to keep the provider abstraction honest — divergences between platforms must be resolved in the adapter, not leaked into metrics, schema, or rendering.

Includes cross-adapter parity tests: equivalent activity recorded in GitHub and GitLab fixture form must normalize to equivalent ActivitySets and produce identical metrics.

Acceptance criteria

• coderepute runs against a GitLab group with a group access token and produces a valid report covering all visible projects
• Identity is bound to the GitLab account ID; coverage stamp behavior matches GitHub's
• Cross-adapter parity tests prove equivalent fixtures yield equivalent ActivitySets and identical metric values
• No GitLab-specific concepts leak into the report schema or renderer

Blocked by

• Full GitHub collaboration metrics (reviews, comments, merge timing, rework) #3
• Org-scoped coverage via org-owned tokens with coverage stamping #5