M20: Native function calling for Claude and OpenAI

[bug-ops]

Overview

Support native tool_use for Anthropic and OpenAI APIs alongside existing text-based bash extraction. Deferred stretch goal — scope only after M19 is stable.

Full plan: .local/plan/m18-tool-orchestration.md (Phase E)

Issues

• feat(llm): extend LlmProvider trait with tool_use support #254 — Extend LlmProvider trait with tool_use support
• feat(llm): add ToolUse and ToolResult message parts #255 — Add ToolUse/ToolResult message parts
• feat(llm): Claude provider native tool_use #256 — Claude provider native tool_use
• feat(llm): OpenAI provider native function calling #257 — OpenAI provider native function calling
• feat(core): dual-mode agent loop for native tool_use #258 — Dual-mode agent loop for native tool_use

Dependencies

• feat(tools): tool registry, file executor, doom-loop detection #246 (M18: tool registry)
• M19: Tool permissions and output overflow #247 (M19: permissions + overflow)