fix(gitea): WriteFile creates via POST, updates via PUT (real contents API)

A live token-scope probe against mathias/ai-sessions revealed gitea's
contents API uses POST to create and PUT (sha required) to update — the
first impl always PUT'd, so creating a new summary 422'd "[SHA]: Required".
The httptest mock had the same wrong assumption. Pick the method by
whether the file exists (GET sha). Token confirmed contents:write
(push:true) by the probe.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/gitea/gitea.go

@@ -109,15 +109,19 @@ func (c *Client) WriteFile(ctx context.Context, repo, path, content string) erro
 		"message": "capture: " + path,
 		"content": base64.StdEncoding.EncodeToString([]byte(content)),
 	}
+	// Gitea contents API: POST creates a new file, PUT updates an existing
+	// one (PUT requires the current sha). Pick by whether the file exists.
+	method := http.MethodPost
 	if sha != "" {
-		payload["sha"] = sha // update in place
+		method = http.MethodPut
+		payload["sha"] = sha
 	}
-	status, body, err := c.request(ctx, http.MethodPut, cpath, payload)
+	status, body, err := c.request(ctx, method, cpath, payload)
 	if err != nil {
 		return err
 	}
 	if status < 200 || status >= 300 {
-		return fmt.Errorf("gitea PUT %s: status %d: %s", cpath, status, strings.TrimSpace(string(body)))
+		return fmt.Errorf("gitea %s %s: status %d: %s", method, cpath, status, strings.TrimSpace(string(body)))
 	}
 	return nil
 }