fix(chroot): detect cargo binaries when CARGO_HOME unset

[Copilot]

Problem

rustc --version fails with exit code 127 in chroot mode when CARGO_HOME environment variable is not set, despite rustc/cargo being installed at ~/.cargo/bin.

Root Cause

In chroot mode with AWF_HOST_PATH set (the common case with sudo), entrypoint.sh only adds Cargo binaries to PATH when AWF_CARGO_HOME is explicitly provided. Many environments install Rust at the standard location without setting CARGO_HOME.

Changes

Added fallback detection in containers/agent/entrypoint.sh when AWF_CARGO_HOME is unset:

if [ -n "${AWF_CARGO_HOME}" ]; then
  echo "export PATH=\"${AWF_CARGO_HOME}/bin:\$PATH\"" >> "/host${SCRIPT_FILE}"
  echo "export CARGO_HOME=\"${AWF_CARGO_HOME}\"" >> "/host${SCRIPT_FILE}"
else
  # Fallback: detect from default location
  echo "[ -d \"\$HOME/.cargo/bin\" ] && export PATH=\"\$HOME/.cargo/bin:\$PATH\"" >> "/host${SCRIPT_FILE}"
fi

This matches the existing pattern for other tools in the fallback PATH construction block.

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[Copilot]

Pull request overview

Fixes Rust tool availability in chroot mode when CARGO_HOME isn’t set by adding a fallback PATH injection for the standard ~/.cargo/bin install location. This improves reliability of host Rust tool usage under sudo/chroot setups where only AWF_HOST_PATH is provided.

Changes:

• Add a fallback branch in entrypoint.sh to prepend "$HOME/.cargo/bin" to PATH when AWF_CARGO_HOME is unset.
• Emit a comment into the generated chroot command script documenting the fallback PATH behavior.

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[Copilot]

The new fallback branch for Rust/Cargo PATH resolution isn’t currently exercised by CI: test-chroot.yml explicitly sets CARGO_HOME and adds $CARGO_HOME/bin to PATH, so AWF_CARGO_HOME is always present and this else-path won’t be covered. Consider adding an integration test (or a CI job variant) that runs chroot with CARGO_HOME unset and PATH not containing ~/.cargo/bin, then asserts rustc --version succeeds to prevent regressions.