docs: consolidate nodeup.md into README and remove the standalone design doc

[dipto0321]

Summary

Removes the internal nodeup.md design doc (1145 lines, 1 commit ever since
scaffolding) and consolidates the user-facing bits into README.md. Then
strips the now-dangling nodeup.md §X references from source, config, docs,
and issue templates so a grep nodeup.md against the tracked tree returns
nothing.

This is pure housekeeping — no code behavior changes, no public API changes,
no CI changes. The user-facing surface of the tool is identical; the README
just covers a few previously-implicit things that the blueprint used to
spell out (compatibility notes, full commit scopes enum, breaking-change
convention).

Changes

README.md

• New "Compatibility notes" subsection after Features. Covers:
  • nvm is a shell function (sourced transparently)
  • Multi-manager install → user is prompted the first time
  • System Node (Homebrew/apt/Windows installer) is detected but cannot
    be upgraded
  • Bundled packages (npm, corepack, npx) are always skipped
    during migration
  • Native addons may need npm rebuild -g after a major Node bump
  • Concurrent runs blocked via ~/.nodeup/nodeup.lock; mid-upgrade
    crash leaves a snapshot that can be restored
• Expanded Contributing. Was a 5-line TL;DR; now covers:
  • Full branch-type list (feat, fix, chore, docs, ci,
    test, refactor)
  • Commit type enum (matches commitlintrc.yml exactly)
  • Commit scope enum (detector, manager, packages, node,
    config, ui, platform, cli, deps, release, ci,
    docs, lint)
  • Breaking-change convention (! marker + BREAKING CHANGE: footer)
  • Squash-merge + delete-branch + tag-driven release
• Removed the 3 dangling See [\nodeup.md`](./nodeup.md)` links.

CHANGELOG.md

• New Unreleased / Changed entry describing the consolidation.
• v0.0.0 entry rewritten to describe the blueprint by topic rather
  than by filename (since the file is gone).

Source / config / docs / templates

All updated to drop See nodeup.md §X cross-refs:

• internal/cli/{config,version,check,list,upgrade,packages}.go — 6 files
• internal/detector/{fnm,nvm,volta,asdf,detector}.go — 5 files
• docs/managers.md, docs/configuration.md
• .commitlintrc.yml (2 comments)
• .goreleaser.yaml (1 trailing comment)
• .github/ISSUE_TEMPLATE/feature_request.md (replaced the
  back-link with the explicit out-of-scope list inline)

nodeup.md

Deleted. .gitignore already lists it, so this is belt-and-suspenders
for anyone cloning from a commit before this PR.

Why now

nodeup.md was useful as scaffolding scaffolding — Phase 0 (repo
foundation) and Phase 1 (detector surface, now complete) referenced it
extensively because there was no other home for the rationale. After
PR #8 landed (8/8 managers merged), every section of the blueprint is
either in the code itself, in the .commitlintrc.yml / .goreleaser.yaml
configs, or has been superseded by CHANGELOG.md. The doc had become
a maintenance liability (any cross-ref update had to touch 11+
files) without adding user value beyond what README.md could carry.

Verification

• go build ./... ✅
• go vet ./... ✅
• go test -race ./... ✅ (all existing tests still pass)
• GOOS=windows go build ./... ✅
• GOOS=windows go vet ./... ✅
• grep -rn "nodeup\.md" --include="*.md" --include="*.go" --include="*.yml" --include="*.yaml" . — only matches in CHANGELOG.md (intentional historical references) and .puku/plans/... (Puku's internal plans, gitignored)
• YAML files parse: .commitlintrc.yml, .goreleaser.yaml ✅
• Commit message validated against .commitlintrc.yml: type docs
  is in type-enum, subject 76 chars (≤ 100), no scope, body lines
  all ≤ 100 chars, no trailing period.

Risk

Effectively zero. Pure docs/comment edits plus file removal. No
production code paths touched. The only runtime surface in the diff is
a reworded comment in internal/cli/version.go and the deleted file
itself.

Rollback

git revert 7aaf6e7 brings nodeup.md back unchanged.

[cocogitto-bot]

✔️ 7aaf6e7 - Conventional commits check succeeded.