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/conformance_test.go

@@ -1,5 +1,3 @@
-//go:build phase2
-
 package gitea_test
 
 import (
@@ -7,19 +5,6 @@ import (
 	"gitea.weiker.me/rodin/review-bot/vcs"
 )
 
-// Compile-time interface conformance assertions.
-// These will verify gitea.Client satisfies vcs interfaces once the Phase 2
-// adapter bridges the method signature gaps:
-//
-//   - PRReader: GetPullRequest returns *gitea.PullRequest (needs *vcs.PullRequest)
-//   - PRReader: GetPullRequestFiles returns []gitea.ChangedFile (needs []vcs.ChangedFile)
-//   - FileReader: GetFileContent lacks ref parameter
-//   - Reviewer: PostReview uses (event, body, comments) instead of vcs.ReviewRequest
-//
-// Remove the phase2 build tag once the adapter is complete.
-var (
-	_ vcs.PRReader   = (*gitea.Client)(nil)
-	_ vcs.FileReader = (*gitea.Client)(nil)
-	_ vcs.Reviewer   = (*gitea.Client)(nil)
-	_ vcs.Identity   = (*gitea.Client)(nil)
-)
+// Compile-time interface conformance assertion.
+// The Adapter (not the raw Client) satisfies the full vcs.Client interface.
+var _ vcs.Client = (*gitea.Adapter)(nil)