fix: remove rbash — too restrictive for practical use

[majiayu000]

Summary

Remove rbash (restricted bash) as the shell used in restricted mode, as it was too restrictive to be useful and prevented normal operations from working.

Context

Restricted mode (--restricted / -r flag) was previously implemented by swapping the user's shell for /bin/rbash, the restricted variant of bash. While this sounds like a security feature, rbash blocks so many basic operations (path execution, directory changes, redirects) that the agent couldn't do anything meaningful. The intended security boundary is better enforced through the existing permissions feature, not at the shell binary level.

Changes

• crates/forge_infra/src/env.rs: Removed the rbash branch from get_shell_path() — restricted mode now falls through to the same shell resolution as unrestricted mode (user's $SHELL or /bin/sh). Updated doc comment to reflect the new semantics.
• crates/forge_infra/src/executor.rs: Removed the rbash override in prepare_command() that was forcing rbash regardless of the configured shell.
• crates/forge_main/src/cli.rs: Updated the --restricted flag description to clarify it enables the permissions feature, not a restricted shell.
• crates/forge_services/src/tool_services/shell.rs: Removed stale doc comment referencing rbash.

Testing

# Verify no compilation errors
cargo check --all-features --workspace

# Run tests
cargo insta test --accept

# Manual: run with --restricted flag and confirm commands execute normally
forge --restricted

Links

• Fixes: [Bug]: Shell commands fail when default shell is set to rbash #2559

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✅ tusharmath
❌ majiayu000
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