Add Copilot repository and path-scoped agent instructions

[Copilot]

The repository had no agent instructions of any kind (no CLAUDE.md, AGENTS.md or copilot-instructions.md), so every agent session rediscovered the umbrella's non-obvious constraints from scratch — or got them wrong. Adds repository-wide custom instructions plus two path-scoped files, all verified against the props, csprojs, workflows, scripts and tests that actually enforce the rules.

.github/copilot-instructions.md

• Overview — facade over three binding repos; two packages × eight variants; no native code and no binding code here; exact-pinned platform packages; $(FFmpegVersion).$(FFmpegKitBindingRevision).
• Build & verify — macOS required (both packages multi-target Android + iOS + macOS, so restoring needs the iOS workload even for Android-only work); .NET 9 per global.json and .NET 10 for the second pack pass; dotnet build FFmpegKit.Net.sln fails on whichever band is missing.
• Conventions — British spelling in prose but American in identifiers/SPDX; Ffmpegkit.Net namespaces; exact [pin] ranges; per-variant obj/bin; the heavily-commented csproj style is load-bearing and should be extended, not stripped.
• CI & release — PR beta publish → release-note merge on master → auto-release tag → guard job → nuget push → GitHub release. Releases skip re-verification because the PR verified the same commit, which is why the guard must stay.
• Hard rules — no native binaries or binding projects, no floating/widening pins, no -beta.* pin reaching a release, no Catalyst, no hand-editing merged nupkgs, licence machinery intact.

.github/instructions/workflows.instructions.md — applyTo: ".github/workflows/*.yml"

Trusted-publishing contract (environment: nuget.org, NUGET_USER as the only secret, id-token: write), verify input semantics, master not main, and why release.yml needs both its workflow_dispatch and push: tags triggers.

.github/instructions/packaging.instructions.md — applyTo: "src/**/*.csproj, Directory.Build.props, build/**"

Pin discipline including the mirror in tests/FFmpegKit.Net.PackageTests/Packages.cs, the SDK band/TFM matrix, per-variant output paths, the merge invariant (a dependency group must carry its <dependency> children, not be synthesised empty), and the deliberately literal MAUI Controls floors.

Notes for review

Two things in-repo are misleading and were deliberately not repeated:

• upstream-drift.yml's REPIN_HINT points at ./build/BumpNativeVersion.sh, which does not exist here.
• The PR beta is derived from FFmpegVersion, i.e. 8.1.2-beta.<pr>.<run> — not the four-part VersionPrefix.