Add reproducible local development matrix

[davidsolc-ai]

Summary

• add a pinned Nix development shell with PHP 8.5, Composer, Just, Docker CLI, ShellCheck, Actionlint, and Alejandra
• use Just as the single project command interface for dependency installation, local CI, focused cells, and the full matrix
• keep local vendor/ user-owned for editor and language-server indexing
• mirror hosted CI in Docker with PHP 8.2, 8.3, and 8.4 on lowest/highest dependencies plus PHP 8.5 on highest dependencies
• reject PHP 8.5/lowest before Docker work because component-model 3.1 emits PHP 8.5 deprecations
• bake source into ephemeral images so container-owned dependency files never affect host permissions
• execute immutable image IDs for concurrent-worktree safety and preserve signal/failure semantics

Development

• direnv allow enters the pinned shell
• just install creates local highest-dependency vendor/
• just ci runs the local suite on PHP 8.5
• just matrix runs all seven isolated compatibility cells
• just cell 8.5 highest runs one cell

Verification

• Nix flake checks on x86_64-linux, aarch64-linux, and aarch64-darwin
• PHP 8.5.8, Composer 2.10.2, Docker 29.6.2 available through direnv
• just install, just ci, and all seven Docker cells pass
• 12 tests pass in every matrix cell
• highest cells pass lint, PHPStan, ECS, Rector, and Tester
• Just formatting, Alejandra, ShellCheck, Actionlint, and Git whitespace checks pass
• HUP, INT, TERM, build-failure, and temporary-file cleanup paths verified