Add Copilot repository instructions and path-scoped instruction files

[Copilot]

The repository had no agent instructions, so every Copilot session had to rediscover the things that are easy to get wrong here: the dual FFmpeg/FFmpegKit version schemes, the checksum-gated native fetch, and the two-pass pack.

Adds .github/copilot-instructions.md plus two path-scoped instruction files. Every command was checked against the script or workflow that runs it (build/resolve-versions.sh was run read-only to confirm both directions and the unmapped-version failure).

.github/copilot-instructions.md (~900 words)

• Overview — class-parse binding, eight variants from one project via FFmpegKitBuildType, native .aar from dev.ffmpegkit-maintained on Maven Central (arthenica retired), and the two version schemes with build/native-versions.tsv as their single source of truth.
• Build & verify — FetchJars → BuildNugets → PackageTests, single-variant pack with FFmpegKitSdkBand, --verify, and the baseline-first order for trying a new upstream line.
• Layout, conventions, CI/release, testing — Ffmpegkit.Droid never moves, build-file comments are load-bearing, British spelling, invariant parsing returning null; PR beta → merged release note → auto-tag → guarded release; PackageTests always, device script when touching Additions/, Transforms/, the .aar wiring or the consumer .targets.
• Hard rules — no committed binaries, no unbaselined or unverified fetch, no version-scheme confusion, keep FFMPEGKIT001/FFMPEGKIT002 on PrepareForBuild, keep the GPL/LGPL split and the non-GPL sample, ./artifacts over nuget.org, no hand-edited merged packages, no bypassing the release guard.
• Closes with the instruction to trust the file and search only where it is incomplete or wrong.

.github/instructions/native-versions.instructions.md

applyTo: "build/native-versions.tsv, build/checksums/**, src/FFmpegKit.Android/Jars/**" — the ordered procedure for adopting a new upstream line: read the FFmpeg version out of the shipped libavcodec.so, record the baseline, add the mapping row, then bump the props.

./build/update-checksums.sh 8.2.0                        # record + commit the baseline first
./src/FFmpegKit.Android/Jars/FetchJars.sh 8.2.0          # refuses to run without one

.github/instructions/workflows.instructions.md

applyTo: ".github/workflows/*.yml" — verify input semantics (true on PRs, false on releases), trusted publishing (environment: nuget.org, id-token: write, only NUGET_USER, push immediately after login), the release guard job, the auto-release → dispatch requirement, Linux runners and the x86_64/KVM/disk constraints on the emulator jobs.

No code, build or workflow behaviour is changed.