feat(nodeup): standardize actionable error messages with hints

[kdy1]

Summary

• standardize all NodeupError messages to <cause>. Hint: <next action>
• keep JSON error envelope schema stable (kind, message, exit_code) while improving message text
• migrate nodeup command/runtime error paths to actionable hints (toolchain/run/show/which/self/installer/release-index/etc.)
• update integration tests for new message wording and hint presence
• update nodeup docs/contracts/public docs to reflect the new error-message behavior

Validation

• cargo fmt --all
• cargo test -p nodeup
• cargo test

Notes

• clap-generated parser errors are intentionally unchanged