Add Copilot CLI instructions for the repository

[benhillis]

Adds .github/copilot-instructions.md so future Copilot CLI sessions in this repo start with the right context.

What's covered

• Repo shape — what the top-level dirs are for (Dockerfile / build-and-export.sh, WSLGd, rdpapplist, WSLDVCPlugin, vendor/, config/, debuginfo/, devops/, package/) and that the WSLDVCPlugin builds separately on Windows.
• Build commands — vendor clone with the critical working-branch caveat (main tracks upstream and won't build), the preferred ./build-and-export.sh flow, the 7 required --build-args and the fail-fast placeholder rules ("" / <unknown> / <current> / unknown rejected; "dev" is the local-fallback sentinel), SYSTEMDISTRO_DEBUG_BUILD=true for debug VHDs, per-component meson build && ninja for iteration, and the .wslconfig systemDistro= swap for try-out.
• Architecture essentials — the Dockerfile is the source of truth; components build in a fixed order (DirectX-Headers → Mesa → PulseAudio → FreeRDP → rdpapplist → Weston → WSLGd), with deliberately minimised option sets (Mesa: only swrast + d3d12, no LLVM/Vulkan; Weston: RDP backend only, rdprail-shell; FreeRDP: server-only, channel GFXREDIR + RDPAPPLIST on; PulseAudio: simple DB, no tests). At runtime wsl.conf boots WSLGd as the user wslg, which launches Weston / PulseAudio / msrdc|mstsc, restarts crashed children, and shares state under /mnt/wslg (SHARE_PATH).
• Conventions — LOG_ERROR / LOG_INFO macros from WSLGd/common.h (not raw fprintf); FreeRDP/WinPR v3-then-v2 dependency fallback in both Meson files; /etc/versions.txt generation + the historical reason for the fail-fast loop; debuginfo/strip_debuginfo.sh per built component with a matching .list file; aggressive runtime-image RPM/file purge in the non-debug path; read-only VHD at runtime → permanent changes belong in the Dockerfile; upstream-component changes belong in the microsoft/*-mirror working branches, not this repo.

Doc-only change, no code or build impact.

[Copilot]

Pull request overview

Adds a new .github/copilot-instructions.md providing Copilot CLI sessions with repository-specific context: layout, build commands (including the vendor/ working-branch requirement and the 7 required --build-args), runtime architecture, and conventions. Doc-only; no code or build impact.

Changes:

• Documents repo layout and component roles (WSLGd, rdpapplist, WSLDVCPlugin, vendor/, etc.).
• Documents build flow: vendor clone, build-and-export.sh, manual docker build args, debug-build flag, per-component Meson commands.
• Documents architecture/runtime flow and conventions (build order, intentional option minimisations, logging macros, FreeRDP v2/v3 fallback, debuginfo split, runtime-prune, read-only rootfs).

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