Installer should surface external Cargo target usage when local disk is tight

[OpenCodeEngineer]

Summary

./install.sh can fail on macOS/Linux with No space left on device when the default codex-rs/target lives on a smaller root volume, even though the user may have a large secondary volume available.

The script already honors CARGO_TARGET_DIR, but the current help/error path does not make that recovery obvious enough.

Desired outcome

• Document using an external CARGO_TARGET_DIR for source builds.
• Improve installer failure output so it suggests moving the build cache/target to another volume.
• Add regression coverage for the guidance path.

Example

CARGO_TARGET_DIR=/Volumes/Dzianis-3/codex-target ./install.sh

[OpenCodeEngineer]

Implemented the installer fix locally and validated it two ways:

• ./scripts/install/test-source-build-install.sh
• real source install with build outputs on /Volumes/Dzianis-3:
  CODEX_INSTALL_DIR=$HOME/.cargo/bin CARGO_TARGET_DIR=/Volumes/Dzianis-3/codex-target TMPDIR=/Volumes/Dzianis-3/codex-tmp CODEX_BUILD_JOBS=1 ./install.sh

The real install completed successfully and produced:

• /Volumes/Dzianis-3/codex-target/release/codex
• /Users/engineer/.cargo/bin/codex

Next step is opening the PR linked to this issue.

[OpenCodeEngineer]

PR opened: #55

It includes the installer fix, docs update, and shell-test coverage for external CARGO_TARGET_DIR usage.