Introduce `include/markdown.h` with retoor copyright header and add `r_process_result_t` struct plus `r_bash_execute_ext` / `r_process_result_free` declarations to `include/bash_executor.h` for advanced async process handling.
- Add agent_set_is_subagent and agent_set_tool_registry to agent API for subagent support
- Add timeout_seconds parameter to r_bash_execute to prevent hanging processes
- Refactor context_manager truncation logic with dynamic ratio-based truncation and total size calculation
- Add messages_to_json_string helper and r_config_get_max_tokens accessor
- Introduce tool_registry_get_specialized for type-filtered tool registries
- Add completion_phrases and passive_phrases detection to agent completion logic
- Suppress spinner in http_client when stderr is not a TTY
- Add comprehensive agent_benchmark.py with TestCase framework and logging
Implement context manager integration that detects API errors indicating context length exceeded, then automatically shrinks message history and retries the request. Add new messages API functions (remove_range, get_object, replace_at) and a new R_ERROR_CONTEXT_TOO_LONG error code to support this functionality.
