feat(plugins): CLI tooling — init, build, test, doctor, dev watch/reload

[MerhiOPS]

Summary

• add volt plugin init, build, test, and doctor as a dedicated volt plugin command group
• scaffold plugin projects, bundle plugin backends, smoke-test them in the real volt-plugin-host, and report compatibility diagnostics
• extend volt dev to discover configured plugin projects, rebuild them on change, and restart plugin host processes on successful rebuilds
• add focused CLI, harness, and doctor tests plus plugin CLI docs

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• pnpm typecheck
• pnpm test

Notes

• cargo test --workspace was rerun standalone after an initial watchdog timing failure while multiple heavy validation commands were running in parallel; the standalone run passed cleanly.