docs: use gh stack for pull request stacks

[wan9chi]

Motivation

The contributor guidance recommends Graphite’s gt command when available, but the project now standardizes on GitHub CLI’s native stacked pull request workflow. Update the instruction so contributors use gh stack to create pull request stacks.

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_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

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[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.40%  [ -3.68% ..  +4.57%]  overhead   +61.03%
dynamic/access             change  -0.20%  [ -1.54% ..  +1.09%]  overhead   +50.84%
static/launch              change  +1.18%  [ -5.94% ..  +8.75%]  overhead  +166.13%
static/access              change  +0.21%  [ -1.11% ..  +1.90%]  overhead  +870.75%

macos

dynamic/launch             change  -0.39%  [ -5.14% ..  +4.82%]  overhead  +217.33%
dynamic/access             change  +0.87%  [ -4.04% ..  +8.00%]  overhead   +29.35%

windows

dynamic/launch             change  +0.22%  [ -5.85% ..  +5.03%]  overhead   +32.96%
dynamic/access             change  -0.17%  [ -1.20% ..  +1.04%]  overhead    +8.14%