ci: add node-prepare action (PM-agnostic)

[ACoolmanTelicent]

Goal

node-prepare replaces yarn-prepare for pnpm repos

Work Completed

• node-prepare composite action: detects PM from lockfile, corepack for pnpm, yarn cache for yarn
• .nvmrc fallback for node version
• javascript-test-feature-branch.yml updated with auto-detect + dual paths

Testing

• CI passes on fe-mono-closed using node-prepare from this branch https://github.com/Telicent-io/fe-mono-closed/actions/runs/26717718170

Out-of-scope

But fast follows

• rm tsc escape hatch (dead code, zero consumers)
• add --frozen-lockfile to yarn path
• Migrate callers off yarn-test-command
• Add lint/format/build gates