feat: add isNub runtime detection

[colinhacks]

Adds isNub to detect Nub, a CLI that augments Node.js through Node's own extension surfaces — not a separate runtime. Under Nub, process.versions.node is set, so detection is additive: isNub is true while isNode stays true and runtime stays "node". It's a layer on Node, not a sibling of Bun/Deno, so it's deliberately not in runtimeChecks or RuntimeName.

The signal is process.versions.nub, Nub's self-identification marker (process.versions.<runtime> convention, cf. .bun, .electron) — mirroring isNode = !!process?.versions?.node. Absent under Nub's --node/NODE_COMPAT passthrough.

Summary by CodeRabbit

• New Features

  • Added a new runtime flag, isNub, to detect Nub environments.
  • Exported isNub from the main package entry point.

• Documentation

  • Updated runtime detection docs/README to include isNub.
  • Clarified that Nub detection is additive, so isNode and runtime remain unchanged.

• Tests

  • Extended the default environment test to verify isNub is false by default.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e330dca2-faea-4dee-b4ed-1481b8cee344

📥 Commits

Reviewing files that changed from the base of the PR and between e7e0577 and ad3f3c7.

📒 Files selected for processing (1)

• README.md

✅ Files skipped from review due to trivial changes (1)

• README.md

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📝 Walkthrough

Walkthrough

Adds an isNub runtime flag based on process?.versions?.nub, re-exports it from the package entry point, updates runtime documentation, and extends the default export test to include isNub: false.

Changes

isNub detection flag

Layer / File(s)	Summary
isNub implementation and export src/runtimes.ts, src/index.ts	Adds exported constant isNub checking process?.versions?.nub and re-exports it from the package entry point.
Tests and documentation test/index.test.ts, AGENTS.md, README.md	Extends the default-environment test to check isNub: false, and documents the additive isNub flag in AGENTS.md and README.md.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: pi0

Related PRs: None identified.

Suggested labels: None identified.

Poem

A tiny flag joins runtime lore,
Nub is seen, while Node stays true,
Docs and tests now speak it too.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding isNub runtime detection.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@README.md`:
- Around line 20-27: The README has two consecutive `> [!NOTE]` blockquotes
separated by a blank line, which markdownlint flags as ambiguous. In the README
section containing the `isNode` and `isNub` notes, remove the empty line between
the two note blocks so they read as separate adjacent blockquotes without extra
separation.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 4a22bfad-e009-4f3b-8444-abeca2dded4f

📥 Commits

Reviewing files that changed from the base of the PR and between 7b6b8b1 and e7e0577.

📒 Files selected for processing (5)

• AGENTS.md
• README.md
• src/index.ts
• src/runtimes.ts
• test/index.test.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.23%. Comparing base (5558f03) to head (ad3f3c7).
⚠️ Report is 70 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #197       +/-   ##
===========================================
+ Coverage   82.08%   95.23%   +13.15%     
===========================================
  Files          10        5        -5     
  Lines         374       63      -311     
  Branches       33       21       -12     
===========================================
- Hits          307       60      -247     
+ Misses         61        2       -59     
+ Partials        6        1        -5     

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