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.

config.example.yaml

@@ -156,6 +156,14 @@ nonstream-keepalive-interval: 0
 #         - "API"
 #         - "proxy"
 
+# Default headers for Claude API requests. Update when Claude Code releases new versions.
+# These are used as fallbacks when the client does not send its own headers.
+# claude-header-defaults:
+#   user-agent: "claude-cli/2.1.44 (external, sdk-cli)"
+#   package-version: "0.74.0"
+#   runtime-version: "v24.3.0"
+#   timeout: "600"
+
 # OpenAI compatibility providers
 # openai-compatibility:
 #   - name: "openrouter" # The name of the provider; it will be used in the user agent and other places.