feat: add pi coding agent provider

[hev]

Add belaykit/pi package — a provider that wraps the pi coding agent CLI as a belaykit.Agent implementation.

What it does

Runs pi --mode json --no-session -p <prompt> and parses the JSONL event stream, mapping pi's events to belaykit's Event types:

pi event	belaykit event
session	EventSystem
turn_start	EventAssistantStart
message_update (text_delta)	EventAssistant
tool_execution_start	EventToolUse
tool_execution_end	EventToolResult
message_end	(accumulates usage/cost/model)
agent_end	EventResult

Pi-specific options

Beyond the standard WithExecutable, WithDefaultModel, WithDefaultEventHandler, WithObservability:

• WithDefaultProvider — set LLM provider (anthropic, openai, google, etc.)
• WithDefaultThinking — set thinking level (off, minimal, low, medium, high, xhigh)
• WithTools / WithNoTools — control which built-in tools are available
• WithExtension — load pi extensions
• WithWorkDir — set the working directory for the pi process

Usage

client := pi.NewClient(
    pi.WithDefaultModel("sonnet:high"),
    pi.WithWorkDir("/path/to/project"),
)

res, err := client.Run(ctx, "List all .ts files in src/")

Token usage

Unlike shelling out to Claude CLI (which reports cost directly in its result event), pi reports per-message usage in message_end events with input/output token counts and cost breakdowns. The provider accumulates these across all messages and passes real numbers to RecordCompletion.

Files

• pi/pi.go — Client struct, Run implementation, JSONL parser
• pi/options.go — ClientOption functions
• pi/models.go — PricingForModel, ContextWindowForModel helpers