feat(cli): add interactive agent REPL with Chat gRPC service

[drew]

Summary

Add a nav agent command that runs an interactive REPL loop on the client, communicates with LLMs through a new Chat gRPC service on the Navigator gateway server, and executes tools by shelling out to nav CLI commands to operate the cluster.

Design

• Agent loop runs client-side. The gRPC Chat service handles single LLM calls only (server-streaming RPC). The client orchestrates multi-turn tool-calling loops.
• Tools shell out to the CLI. The agent executes tools by running nav <subcommand> <args> as subprocesses (e.g., nav sandbox list, nav cluster status). No direct gRPC calls for tools.
• Binary resolution: Use std::env::current_exe() by default, with NAV_AGENT_CLI env var override for dev.
• Cluster context pinning: Every shell-out includes --cluster <name> so subcommands target the correct cluster.
• Provider-agnostic tool calling protocol. Proto defines its own Tool, ToolCall, ChatMessage messages. The server translates to/from OpenAI or Anthropic format based on the resolved inference route's protocol.
• Route selection via routing_hint = "agent". The operator creates an inference route with routing_hint: agent. The Chat service filters routes using the existing list_sandbox_routes function.
• Streaming responses. ChatStream is a server-streaming RPC that streams ContentDelta events as tokens arrive via SSE, then sends a final ChatMessage with any tool_calls.

Components

Proto (proto/chat.proto)

• Chat service with ChatStream server-streaming RPC
• ChatStreamRequest, ChatMessage, Tool, ToolCall, ChatStreamEvent, ContentDelta, ChatError messages

Server (crates/navigator-server/src/chat/)

• ChatService — gRPC implementation with route resolution, protocol detection, spawned streaming task
• openai.rs — OpenAI format translation and SSE streaming with tool call accumulation
• anthropic.rs — Anthropic format translation (system prompt extraction, tool_use/tool_result blocks) and SSE streaming
• Multiplexer extended from 2 to 3 gRPC services

CLI (crates/navigator-cli/src/agent.rs)

• 9 tools: cluster_status, sandbox_list, sandbox_get, sandbox_create, sandbox_delete, sandbox_logs, provider_list, provider_get, inference_route_list
• Binary resolution with NAV_AGENT_CLI env var override
• Interactive REPL using rustyline with tool-calling loop
• --routing-hint and --system-prompt CLI arguments

Acceptance Criteria

• nav agent starts an interactive REPL
• Agent can call tools and feed results back to the LLM
• Both OpenAI and Anthropic backends are supported
• Streaming token output works correctly
• cargo check, cargo clippy, cargo test, and cargo fmt all pass

[drew]

Will add to roadmap