devenv: Deduplicate kani setup and devcontainer configs

[henrywang]

Move gcc installation into install-kani.sh so the distro-specific package manager command isn't duplicated across all three Containerfiles. The script auto-detects dnf vs apt-get.

Add devcontainer-generate and devcontainer-validate recipes to generate per-OS devcontainer.json files from the default template and validate they stay in sync.

Assisted-by: Claude Code (Opus 4.6)

[gemini-code-assist]

Summary of Changes

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This pull request refines the development environment setup by addressing two key areas: streamlining Kani's dependency installation and improving devcontainer configuration management. By moving gcc installation into a shared script, it eliminates duplication across different container images. Additionally, new just commands provide a robust mechanism to generate and validate OS-specific devcontainer configurations, ensuring consistency and ease of maintenance.

Highlights

• Kani Setup Deduplication: Centralized the installation of gcc (a dependency for Kani) into the install-kani.sh script, removing redundant package manager commands from individual Containerfiles.
• Devcontainer Configuration Management: Introduced new just recipes, devcontainer-generate and devcontainer-validate, to automate the generation and synchronization of per-OS devcontainer.json files from a single template, ensuring consistency across environments.

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[gemini-code-assist]

Code Review

This pull request effectively deduplicates the gcc installation logic by centralizing it in install-kani.sh and introduces just recipes for managing devcontainer configurations, which improves maintainability. The changes are well-structured. I have a couple of suggestions to enhance the robustness and scripting style of the new additions.