docs: document available installation paths

[rogerchappel]

Summary

• stop claiming that ledgerpet is available from the npm registry
• document the source-checkout path and conditional versioned GitHub release artifact path
• avoid npx registry resolution in the quickstart
• add an offline package-install smoke that exercises ledgerpet --help and a fixture inspection
• fix direct execution through npm's .bin/ledgerpet symlink, exposed by the new smoke

Commits

• 20fec60 test: verify installed package CLI offline — resolves the installed bin symlink correctly and adds an offline local-tarball documentation smoke to the release gate
• ed206b5 docs: describe available installation paths — updates installation and quickstart guidance and runs the documentation smoke in CI

Verification

• npm ci
• npm run check
• npm test (22 passed)
• npm run build
• npm run smoke
• npm run docs:smoke
• npm run package:smoke
• npm run release:check
• bash scripts/validate.sh
• git diff --check origin/main..HEAD

Risk Level

• Low
• Medium
• High

Rollback Plan

Revert the two commits; no published packages, data migrations, or external state are involved.

Human Decision Needed

• Maintainer review