feat(pi): add thinking option for reasoning level control

[jerome-benoit]

Summary

Expose pi's --thinking CLI flag through PiOptions, matching the effort option on claudeCode/codex and variant on opencode.

Changes

• src/AgentProvider.ts: Added thinking?: "off" | "minimal" | "low" | "medium" | "high" | "xhigh" to PiOptions and wired --thinking flag in buildPrintCommand.
• src/AgentProvider.test.ts: Added 4 tests covering presence, absence, empty options, and all 6 thinking levels.

Usage

import { pi } from "@ai-hero/sandcastle";

// Without thinking (unchanged behavior)
pi("claude-sonnet-4-6");

// With thinking (new)
pi("claude-sonnet-4-6", { thinking: "high" });
// → pi -p --mode json --no-session --thinking high --model 'claude-sonnet-4-6'

Design decisions

• Typed union (not string): Pi has a fixed, documented set of 6 levels (off, minimal, low, medium, high, xhigh). Using a union provides compile-time safety — same pattern as claudeCode and codex.
• No shell-escaping needed: Values come from a typed union of known-safe alphanumeric strings — same approach as claudeCode's effort.
• buildInteractiveArgs unchanged: Pi's TUI manages thinking level via Shift+Tab — no need to pass it on the interactive command line.

Closes #583

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[Copilot]

Pull request overview

This PR exposes pi’s --thinking reasoning level flag via a new PiOptions.thinking option, aligning pi with the existing “reasoning control” options supported by other agent providers.

Changes:

• Added thinking?: "off" | "minimal" | "low" | "medium" | "high" | "xhigh" to PiOptions.
• Updated pi().buildPrintCommand() to append --thinking <level> when configured.
• Added unit tests to verify --thinking is included/omitted correctly and supports all levels.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/AgentProvider.ts	Adds the thinking option to PiOptions and wires it into the generated pi print command.
src/AgentProvider.test.ts	Adds coverage for the new thinking flag behavior and supported levels.

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