feat: add cached source installer

[dzianisv]

Summary

• add a repo-tracked source-building ./install.sh for fast local installs of the Codex CLI
• replace the old mtime-only rebuild decision with a build stamp and content fingerprint so unchanged files do not trigger rebuilds
• enable incremental release builds by default in fast mode, honor CARGO_TARGET_DIR, and add a shell test that verifies cache reuse and rebuild behavior

Testing

• sh scripts/install/test-source-build-install.sh
• sh -n install.sh scripts/install/test-source-build-install.sh
• sh install.sh --help

Closes #22

[dzianisv]

Superseded by #35, which now carries this branch's changes on the consolidated integration branch.