fix: keep CLI version aligned with package metadata - #27

Summary

• derive promptdiff --version from the packaged package.json version
• verify built and installed tarball CLIs against package metadata, including a non-0.1.0 fixture and a failing mismatch probe
• document the single-source release version procedure

Commits

• 09b1cbe test: define CLI package version contract
• 7ea0929 fix: derive CLI version from package metadata
• 49ee8b3 docs: simplify release version updates

Verification

• npm ci
• npm run release:check (30 tests, typecheck, build, smoke, tarball install/version checks)
• git diff --check

The regression fixture supplies package version 9.8.7 and verifies both the built CLI and installed tarball CLI report it. The mismatch probe verifies 9.8.7 versus 0.1.0 exits nonzero.