fix(tools,gitea): alias repo/index args and reject empty path segments

Per-repo MCP tools 404'd while owner-level tools worked (#36). Root cause was
a parameter-name contract mismatch, not a routing fault: every per-repo tool
declares the repo identifier as 'name' (and issue/PR index as 'number'), but
every caller — the claude.ai connector, LLMs primed on gitea's own API — sends
'repo' and 'index'. The unmatched fields zero-valued the upstream path segment,
producing '/api/v1/repos/{owner}//...', which gitea answers with its generic
api-404 whose body points at /api/swagger. That swagger pointer is gitea boiler-
plate, not a misroute — the MCP dispatch was correct all along.

Two layers of defence:
- parseArgs aliases repo->name and index->number (explicit canonical wins;
  alias key left intact so pr_merge's real 'index' field is unaffected). Kills
  the recurrence by accepting the idiomatic argument names.
- the gitea client rejects any path with an empty segment before the HTTP call,
  returning ErrValidation instead of forwarding a malformed path and surfacing
  gitea's opaque swagger-404. Kills the silent-misleading-error class.

Closes #36

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

internal/gitea/client.go

@@ -3,8 +3,10 @@ package gitea
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/hashicorp/golang-lru/v2/expirable"
@@ -40,7 +42,21 @@ func (c *Client) DefaultBranch(ctx context.Context, owner, name string) (string,
 	return repo.DefaultBranch, nil
 }
 
+// hasEmptySegment reports whether the path portion (before any query string)
+// contains an empty segment ("//"), which means an owner or repo path
+// parameter was empty. Forwarding it upstream yields gitea's opaque
+// /api/swagger 404 (#36), so callers reject it locally instead.
+func hasEmptySegment(path string) bool {
+	if i := strings.IndexByte(path, '?'); i >= 0 {
+		path = path[:i]
+	}
+	return strings.Contains(path, "//")
+}
+
 func (c *Client) doOnce(ctx context.Context, method, path string, body []byte) ([]byte, int, error) {
+	if hasEmptySegment(path) {
+		return nil, 0, fmt.Errorf("%w: upstream path %q has an empty owner or repo segment", ErrValidation, path)
+	}
 	var reader io.Reader
 	if body != nil {
 		reader = bytes.NewReader(body)
@@ -107,6 +123,9 @@ type rawResponse struct {
 }
 
 func (c *Client) doRaw(ctx context.Context, method, path string, body []byte) (*rawResponse, error) {
+	if hasEmptySegment(path) {
+		return nil, fmt.Errorf("%w: upstream path %q has an empty owner or repo segment", ErrValidation, path)
+	}
 	var reader io.Reader
 	if body != nil {
 		reader = bytes.NewReader(body)