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

vcs/check_test.go

@@ -1,5 +1,3 @@
-//go:build phase2
-
 package vcs_test
 
 import (
@@ -7,21 +5,7 @@ import (
 	"gitea.weiker.me/rodin/review-bot/vcs"
 )
 
-// Compile-time assertion: documents the gap between gitea.Client and vcs.Client.
-// Guarded by the "phase2" build tag — enable once the Gitea adapter bridges these gaps:
-//
-//  1. PostReview signature mismatch:
-//     gitea.Client:  PostReview(ctx, owner, repo, number, event, body string, comments []gitea.ReviewComment)
-//     vcs.Reviewer:  PostReview(ctx, owner, repo, number, req vcs.ReviewRequest)
-//
-//  2. GetFileContent signature mismatch:
-//     gitea.Client:  GetFileContent(ctx, owner, repo, filepath string)  [no ref; uses default branch]
-//     vcs.FileReader: GetFileContent(ctx, owner, repo, path, ref string)
-//     (gitea.Client has GetFileContentRef for the ref variant)
-//
-//  3. ReviewComment type mismatch:
-//     gitea.ReviewComment uses NewPosition int64 (Gitea line-number convention)
-//     vcs.ReviewComment uses Position int (GitHub diff-position convention)
-//
-// The Gitea adapter (Phase 2) will wrap gitea.Client to bridge these gaps.
-var _ vcs.Client = (*gitea.Client)(nil)
+// Compile-time assertion: the gitea.Adapter satisfies vcs.Client.
+// (The raw gitea.Client does NOT satisfy vcs.Client due to signature differences;
+// the Adapter bridges them.)
+var _ vcs.Client = (*gitea.Adapter)(nil)