Update reference to upstream direnv-hook.sh file

[Copilot]

This PR adapts to the breaking change introduced in numtide/prj-spec#8, where the contrib/direnv file has been renamed to contrib/direnv-hook.sh.

The upstream change refactored the prj-spec direnv integration by:

• Renaming contrib/direnv to contrib/shell-hook.sh (generic shell hook)
• Creating a new contrib/direnv-hook.sh that sources the shell hook and adds direnv-specific functionality

This maintains the same functionality for direnv users while providing a cleaner separation between generic shell integration and direnv-specific features.

Changes Made

• Updated the fetchurl URL from contrib/direnv to contrib/direnv-hook.sh
• Updated the commit SHA to reference the PR branch: 4727ff5b89020912ead606c9ebbbe3e94d4a9f50
• Updated the SHA256 hash to match the new file content: sha256-iTOGxlNyuWM5w581kGpzTU1joX1UUouNaQKQ63LM+6Y=

The changes are minimal (only 2 lines) and maintain full backward compatibility for existing users. All functionality remains the same.

Fixes #6.

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