feat: always post fresh review, supersede old with collapsed body

Closes #34

- Remove reviewUnchanged() skip logic — every push gets a fresh review
- Remove edit-in-place (PATCH same body) — always POST new
- Supersede old review: PATCH with struck-through banner + collapsed
  original body in <details> for historical reference
- Add commit footer to every review: 'Evaluated against <sha>'
- Remove --update-existing flag (no longer needed)
- Add CommitID field to Review struct
- Add TestBuildSupersededBody tests

gitea/client.go

@@ -316,13 +316,14 @@ func (c *Client) GetAllFilesInPath(ctx context.Context, owner, repo, path string
 
 // Review represents a pull request review from the Gitea API.
 type Review struct {
-	ID   int64  `json:"id"`
-	Body string `json:"body"`
-	User struct {
+	ID       int64  `json:"id"`
+	Body     string `json:"body"`
+	User     struct {
 		Login string `json:"login"`
 	} `json:"user"`
-	State string `json:"state"`
-	Stale bool   `json:"stale"`
+	State    string `json:"state"`
+	Stale    bool   `json:"stale"`
+	CommitID string `json:"commit_id"`
 }
 
 // ListReviews returns all reviews on a pull request.