fix(tools): reject case-variant arg keys and ref-shaped branch names

parseArgs now refuses, before any request: two keys equal under
strings.EqualFold in any object (exact duplicates included), and a
top-level key that differs from a schema field only in case. Without
this, encoding/json's case-insensitive last-wins match let
{"branch":"feat/x","Branch":"main"} write to main while a client-side
hook read feat/x.

file_write_branch (branch) and tbd_ship (branch and base) validate the
branch name: ^[A-Za-z0-9][A-Za-z0-9._/-]*$ plus no refs/ prefix, HEAD,
"..", "//", "/.", trailing "/" or ".", or ".lock". refs/heads/main
otherwise created refs/heads/refs/heads/main and wedged API writes to
main. tbd_ship's base is the protection-probe key, so a refs/-prefixed
base would also have read as unprotected.

Closes #65

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/tools/file_write_branch.go

@@ -63,6 +63,9 @@ func (t *FileWriteBranch) Call(ctx context.Context, raw json.RawMessage) (json.R
 	if args.Branch == "" {
 		return nil, fmt.Errorf("branch is required: %w", gitea.ErrValidation)
 	}
+	if err := ValidateBranchName(args.Branch); err != nil {
+		return nil, err
+	}
 	if args.Message == "" {
 		return nil, fmt.Errorf("message is required: %w", gitea.ErrValidation)
 	}