fix(docker): revert NODE_RED_CLI_DEFAULT_USERDIR; add /data auto-probe + --docker-userdir override

[tbrandenburg]

Issues fixed

Closes #33

Summary

Reverts the NODE_RED_CLI_DEFAULT_USERDIR env-var convention (#31/#32), which required community image authors to opt in and was confirmed dead-on-arrival for the motivating agentic-workflow-dev-env image.

Replaces it with:

• A best-effort /data auto-probe (sandbox entrypoint only, never on the host) that validates node_modules/* and node_modules/@*/* (following symlinks — real npm installs often symlink packages) for a package.json declaring a "node-red" key before trusting the path.
• An explicit --docker-userdir <path> CLI flag as a reliable, zero-guessing override for any image convention.
• A 4-level userDir precedence: --user-dir > --docker-userdir > auto-probed /data > ephemeral default.
• A clearer --node-modules preflight error when combined with --docker-userdir/the auto-probe without an explicit --user-dir.
• Updated README (auto-probe + flag docs, agentic-workflow-dev-env example now uses the auto-probe with zero flags).

A real bug was caught during e2e validation and fixed before merge: the initial Dirent.isDirectory()-based scan missed symlinked npm packages, which is exactly how the real agentic-workflow-dev-env image installs @tbrandenburg/node-red-agents — the probe silently failed against the actual motivating image until fixed to stat (following symlinks) instead of trusting Dirent flags.

Files changed

• src/run-envelope.js — new resolveContainerDefaultUserDir (the /data probe) and resolveEffectiveUserDir (4-level precedence), replacing the reverted env-var logic.
• bin/node-red-cli-sandbox-entry.js — sets probeContainerDefault: true (auto-probe is sandbox-only).
• bin/node-red-cli.js — new --docker-userdir <path> flag, updated help text, extended --node-modules guard.
• test/unit/run-envelope.unit.test.js — unit tests for the probe (true/false positive, symlinks, absent baseDir) and the 4-level precedence.
• test/integration/docker.integration.test.js — real Docker integration tests: auto-probe true/false positive, --docker-userdir override, --user-dir precedence.
• test/integration/run-envelope.integration.test.js — dockerUserDir option equivalents.
• test/e2e/agentic-workflow-dev-env.e2e.test.js (new) — real, non-mocked e2e test against the actual ghcr.io/tbrandenburg/agentic-workflow-dev-env:latest image, proving the auto-probe discovers the real symlinked package with zero flags. Skips gracefully if Docker/network is unavailable.
• README.md — replaced env-var docs with auto-probe + --docker-userdir docs.

Validation commands run

• make format — clean (Prettier)
• make lint — clean (ESLint)
• make test (node --test) — 141/141 passing, 0 failed, 0 skipped
• make ci (format + lint + test + audit, via pre-push hook) — all green, npm audit: 0 vulnerabilities

E2E coverage

• New test/e2e/agentic-workflow-dev-env.e2e.test.js: builds a throwaway image derived from the real ghcr.io/tbrandenburg/agentic-workflow-dev-env:latest + this branch's own bin/src, confirms --docker <image> (no flags) discovers the real, symlinked @tbrandenburg/node-red-agents package via the /data auto-probe. Ran successfully in this environment (Docker + ghcr.io reachable).
• Existing Docker integration tests extended with 4 new real-container cases covering both probe outcomes and the --docker-userdir override + precedence.

Risks / follow-ups

• Breaking change for anyone who adopted the very recently released NODE_RED_CLI_DEFAULT_USERDIR — acceptable per the issue (release-notes-worthy, but intentional).
• --docker-userdir is documented as ignored outside --docker mode and only threaded into the --docker envelope path, matching its stated container-path-override semantics.
• No unrelated bugs found beyond the symlink issue above, which was fixed as part of this change (required for the auto-probe to work against the real motivating image).