feat(vcs): Gitea adapter with diff-position translation

Implements the Gitea adapter (gitea.Adapter) that satisfies vcs.Client.

Key components:
- gitea/adapter.go: Adapter struct wrapping *Client with all vcs.Client methods
- gitea/position.go: BuildPositionToLineMap for diff-position → line translation
- gitea/adapter_test.go: Tests for all mapping methods and event translation
- gitea/position_test.go: Tests for position translation edge cases

Translation details:
- ReviewEvent: APPROVE → APPROVED (Gitea-native)
- PostReview: fetches diff, builds position map, translates each comment
- Deletion-targeted positions map to nearest non-deletion line below
- All field-mapping methods tested (GetPullRequest, GetPullRequestFiles,
  ListReviews, GetCommitStatuses, ListContents)

Also:
- Added Base field to gitea.PullRequest struct
- Updated conformance tests to assert Adapter (not raw Client) satisfies vcs.Client
- Removed phase2 build tag from conformance tests

Closes #79

gitea/client.go

@@ -86,6 +86,9 @@ type PullRequest struct {
 		Sha string `json:"sha"`
 		Ref string `json:"ref"`
 	} `json:"head"`
+	Base struct {
+		Ref string `json:"ref"`
+	} `json:"base"`
 }
 
 // CommitStatus represents a single CI status entry.