Windows shell integration: auto-inject pwsh + cmd, prompt-ready signal

[deblasis]

Auto-injected OSC 133 shell integration for the native Windows shells, plus a prompt-ready signal the app layer can react to.

What it does:

• pwsh / PowerShell 5.1: auto-injected at launch (-NoExit -Command ". '<ghostty.ps1>'"), no $PROFILE edit. The user's profile still loads first, then the integration wraps the final prompt. Reuses the existing ghostty.ps1. Explicit user commands (pwsh -c ...) are left untouched.
• cmd: integration via the PROMPT env var, emitting OSC 133;A / 133;B and OSC 9;9 cwd. Preserves a user-set PROMPT. No C/D marks (cmd has no command hooks).
• New payload-less prompt_ready apprt action, dispatched on OSC 133;B (mirrors command_finished on 133;D), surfaced to C# as TerminalControl.PromptReady.
• shell-integration config now resolves/accepts powershell and cmd.

Notes:

• TerminalControl.PromptReady has no consumer in this repo yet; it's the hook a downstream UI feature subscribes to.
• The .direct launch form for pwsh is deliberate so the dot-source argument survives Windows command re-parsing; bare pwsh still resolves via PATH (CreateProcessW(null, ...)).

Testing:

• Zig: full zig build test green, including the ghostty.h action-enum ABI self-check and new setupPowerShell / setupCmd / detectShell unit tests.
• C#: ABI ordinal pin for PromptReady + full test suite (1156 passing); native dll + app compile clean.
• Cross-platform (macOS/Linux) compile of the new enum variants to be confirmed in CI.