fix(install): honor CARGO_HOME/RUSTUP_HOME + document env-overridable install paths

[agentik-os]

Quoi

install.sh hardcodait source "$HOME/.cargo/env", ce qui ignore un CARGO_HOME défini par l'hôte et force la toolchain Rust dans ~/.cargo. Ce PR :

• honore CARGO_HOME (et RUSTUP_HOME, déjà respecté par rustup) ;
• documente que OMEGA_DIR / INSTALL_DIR / OMEGA_SRC / CARGO_HOME / RUSTUP_HOME sont overridables par env, pour qu'un hôte route ses chemins d'install (setups XDG / home propre, installs multi-users partagées) sans forker le script.

Pourquoi

Permet un rangement propre côté hôte (ex. serveur multi-user où les caches/toolchains vont hors $HOME) sans toucher au script. Aucun chemin spécifique à un hôte n'est hardcodé : OmegaOS reste générique.

Parité d'install (L0)

Comportement identique quand les vars ne sont pas définies (défauts inchangés) → un git clone && ./install.sh standard se comporte exactement pareil. bash -n OK.

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[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between c09b845 and d4fd8d8.

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• install.sh

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Installer now supports customizable installation paths via environment variables, providing greater flexibility for deployments across different system configurations.
  • Improved Rust toolchain setup that properly respects custom CARGO_HOME environment variable overrides, ensuring consistent initialization behavior.

Walkthrough

The installer script is updated to document that installation paths are configurable via environment variables (OMEGA_DIR, INSTALL_DIR, OMEGA_SRC, and standard Rust toolchain variables), and Cargo environment sourcing now respects the CARGO_HOME override rather than always using the hardcoded default.

Environment variable configuration

Layer / File(s)	Summary
Install path documentation and Cargo environment sourcing install.sh	Documentation comments clarify environment variable overrides for install directories and Rust toolchain paths. Cargo environment sourcing respects CARGO_HOME configuration, defaulting to $HOME/.cargo when unset.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

Poem

🐇 A rabbit configures its hutch with care,
Environment paths defined, declared with flair,
No hardcoded homes, just fallbacks so fair,
Each tool finds its place in the mountain air! 🏔️

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