fix: update Claude masquerading headers and make them configurable

Update hardcoded X-Stainless-* and User-Agent defaults to match
Claude Code 2.1.44 / @anthropic-ai/sdk 0.74.0 (verified via
diagnostic proxy capture 2026-02-17).

Changes:
- X-Stainless-Os/Arch: dynamic via runtime.GOOS/GOARCH
- X-Stainless-Package-Version: 0.55.1 → 0.74.0
- X-Stainless-Timeout: 60 → 600
- User-Agent: claude-cli/1.0.83 (external, cli) → claude-cli/2.1.44 (external, sdk-cli)

Add claude-header-defaults config section so values can be updated
without recompilation when Claude Code releases new versions.

internal/config/config.go

@@ -90,6 +90,10 @@ type Config struct {
 	// ClaudeKey defines a list of Claude API key configurations as specified in the YAML configuration file.
 	ClaudeKey []ClaudeKey `yaml:"claude-api-key" json:"claude-api-key"`
 
+	// ClaudeHeaderDefaults configures default header values for Claude API requests.
+	// These are used as fallbacks when the client does not send its own headers.
+	ClaudeHeaderDefaults ClaudeHeaderDefaults `yaml:"claude-header-defaults" json:"claude-header-defaults"`
+
 	// OpenAICompatibility defines OpenAI API compatibility configurations for external providers.
 	OpenAICompatibility []OpenAICompatibility `yaml:"openai-compatibility" json:"openai-compatibility"`
 
@@ -117,6 +121,15 @@ type Config struct {
 	legacyMigrationPending bool `yaml:"-" json:"-"`
 }
 
+// ClaudeHeaderDefaults configures default header values injected into Claude API requests
+// when the client does not send them. Update these when Claude Code releases a new version.
+type ClaudeHeaderDefaults struct {
+	UserAgent      string `yaml:"user-agent" json:"user-agent"`
+	PackageVersion string `yaml:"package-version" json:"package-version"`
+	RuntimeVersion string `yaml:"runtime-version" json:"runtime-version"`
+	Timeout        string `yaml:"timeout" json:"timeout"`
+}
+
 // TLSConfig holds HTTPS server settings.
 type TLSConfig struct {
 	// Enable toggles HTTPS server mode.