Make full Rust CI manual-only and add local Cargo cache helper

[cbusillo]

Summary

• make rust-ci-full.yml genuinely workflow_dispatch only so it matches the fork workflow strategy docs
• add scripts/local/cargo-build-env.sh for opt-in local Cargo target caching under /Volumes/Developer-Artifacts/local/codex-lab/...
• expose the helper via just local-cargo-env
• teach local cleanup to report/remove that artifact-volume Cargo cache unless --keep-local-cargo-cache is passed

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/rust-ci-full.yml"); puts "workflow yaml ok"'
• actionlint .github/workflows/rust-ci-full.yml || true (reports pre-existing shellcheck warnings in untouched script blocks around lines 129 and 144)
• git diff --check
• scripts/local/cargo-build-env.sh
• CODEX_LAB_CARGO_TARGET_NO_MKDIR=1 scripts/local/cargo-build-env.sh
• just local-cargo-env
• python3 -m unittest tools.codex-exec-harness.test_local_cleanup
• just test-local-scripts