feat(tools): file_read with default-branch resolution

Adds GetFileContents to the gitea client and a file_read MCP tool.
When ref is omitted, the tool resolves the repo default_branch via
GetRepo before fetching contents. Decoded content capped at 1 MiB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gitea/files.go

@@ -0,0 +1,34 @@
+package gitea
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+type FileContents struct {
+	Path     string `json:"path"`
+	Sha      string `json:"sha"`
+	Size     int64  `json:"size"`
+	Content  string `json:"content"`
+	Encoding string `json:"encoding"`
+}
+
+func (c *Client) GetFileContents(ctx context.Context, owner, repo, path, ref string) (*FileContents, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", owner, repo, path)
+	if ref != "" {
+		p += "?ref=" + ref
+	}
+	body, status, err := c.GetJSON(ctx, p)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var fc FileContents
+	if err := json.Unmarshal(body, &fc); err != nil {
+		return nil, err
+	}
+	return &fc, nil
+}