Support for directory-based auto-activation (direnv shell hook equivalent)

[schickling]

Problem

A key feature of direnv is its shell hook (eval "$(direnv hook fish)" etc.) which automatically activates/deactivates environments when cd-ing into/out of project directories. This is essential for seamless developer experience — the environment just works without manually running any command.

As devenv moves toward replacing direnv (#2061), an equivalent mechanism is needed: automatic environment activation/deactivation triggered by directory changes.

Expected behavior

Devenv should provide a shell hook (for bash, zsh, and fish) that:

1. Detects when the user enters a directory with a devenv.nix / devenv.yaml
2. Automatically activates the devenv environment (ideally with background evaluation, as noted in Replace direnv with Rust #2061)
3. Deactivates/restores the previous environment when leaving the directory
4. Works with nested projects (inner project takes precedence)

This is the main remaining blocker for fully replacing direnv with devenv.

Related

• Replace direnv with Rust #2061 — Replace direnv with Rust
• Activate profiles automatically based on folder location #2177 — Activate profiles automatically based on folder location

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This issue was created on behalf of @schickling

[domenkozar]

To implement this we need something like directory-auto-activation = true so we know which devenvs should just enter in case you have a monorepo.