feat: accept string or number for index parameters (#121)

This change makes index parameters more flexible by accepting both
numeric and string values. LLM agents often pass issue/PR indices
as strings (e.g., "123") since they appear as string identifiers in
URLs and CLI contexts. The implementation:

- Created pkg/params package with GetIndex() helper function
- Updated 25+ tool functions across issue, pull, label, and timetracking operations
- Improved error messages to say "must be a valid integer" instead of misleading "is required"
- Added comprehensive tests for both numeric and string inputs

This improves UX for MCP clients and LLMs while maintaining backward
compatibility with existing numeric callers.

Fixes: #121

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pkg/params/params.go

@@ -0,0 +1,33 @@
+package params
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// GetIndex extracts an index parameter from MCP tool arguments.
+// It accepts both numeric (float64 from JSON) and string representations.
+// This provides better UX for LLM callers that may naturally use strings
+// for identifiers like issue/PR numbers.
+func GetIndex(args map[string]interface{}, key string) (int64, error) {
+	val, exists := args[key]
+	if !exists {
+		return 0, fmt.Errorf("%s is required", key)
+	}
+
+	// Try float64 (JSON number type)
+	if f, ok := val.(float64); ok {
+		return int64(f), nil
+	}
+
+	// Try string and parse to integer
+	if s, ok := val.(string); ok {
+		i, err := strconv.ParseInt(s, 10, 64)
+		if err != nil {
+			return 0, fmt.Errorf("%s must be a valid integer (got %q)", key, s)
+		}
+		return i, nil
+	}
+
+	return 0, fmt.Errorf("%s must be a number or numeric string", key)
+}