docs: use the available release artifact

[rogerchappel]

Summary

• replace the unavailable npm-registry install command with the executable v0.1.0 GitHub release tarball path
• clarify that installed CLI/API use works from that artifact while repository development uses npm ci
• correct release recovery guidance so a GitHub release is not treated as proof of npm availability
• guard the supported distribution URL and release-state wording in the release contract

Commits

• 1c409ee test: guard supported installation guidance
• 847d20b docs: use the available release artifact

Verification

• npm ci
• npm run release:check (37 tests; syntax, build, CLI smoke, packed-package smoke, release contract)
• clean temporary-project install with npm install https://github.com/rogerchappel/plainforge/releases/download/v0.1.0/plainforge-0.1.0.tgz
• installed plainforge --help
• installed public API import and conversion assertion
• git diff --check origin/main...HEAD

All commit authors and committers are Roger Chappel <miscanalysis@gmail.com>.

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Update README installation and release-recovery guidance to use the available v0.1.0 GitHub release tarball while independently checking npm availability; add a release-contract regression test (2 files, +32/-14).

Checks run: GitHub CI test SUCCESS; Release dry run dry-run SUCCESS; worker verification npm ci, npm run release:check (37/37 tests, syntax, build, CLI/package smoke, release contract), documented clean-install, CLI help, public API import/conversion, and git diff --check passed.

Rebased/CI-repaired: No.

Verified head SHA: 847d20b3f8d980d29a1bdf8dc81d3c66626d15ed.