#2416 Fix Agrico alignment invalid output

[randrag]

Summary

• Fixes Agrico MatchArrowAlignment so an aligned inline arm is only used when the aligned form still fits on the line.
• Falls back to a natural multiline match arm when alignment padding would make the body wrap invalidly.
• Avoids the inline-first-field union payload layout for anonymous record payloads.
• Adds neutral regression tests for the match-arrow and anonymous-record union-case cases from AgricoZA/Ops#2416.
• Moves canonical agent guidance to AGENTS.md and leaves CLAUDE.md as a pointer.
• Bumps package version to 8.0.0-alpha-012-agrico-008.

Verification

• dotnet test src/Fantomas.Core.Tests --filter "FullyQualifiedName~AgricoMatchArrowAlignmentTests" --no-restore
• dotnet test src/Fantomas.Core.Tests --filter "FullyQualifiedName~AgricoUnionCaseAlignmentTests" --no-restore
• dotnet test src/Fantomas.Core.Tests --no-restore
• dotnet fantomas src docs build.fsx
• dotnet fsi build.fsx -- -p Analyze
• dotnet fsi build.fsx

Package published: fantomas 8.0.0-alpha-012-agrico-008.