Rename js/ to pkg-js/

[schloerke]

Motivation

js/ is becoming a published package (@posit/shinyreact, decided in decisions/2026-08-17-js-distribution.md), making it a sibling of pkg-py/ and pkg-r/ rather than internal build tooling. Rename it to pkg-js/ to match the language-named pkg-* convention.

This is a standalone mechanical PR that should land before the substantive JS-distribution work (config script tag, dual-output build, npm publishing).

Touchpoints

• git mv js pkg-js
• Makefile — js-* targets (js-setup, js-build, js-build-watch, js-lint) and the update-dist copy paths (js/dist/ → pkg-py/src/shinyreact/www/, pkg-r/inst/lib/shiny/)
• .gitignore — the !js/dist/ whitelist entries
• examples/09-hmr/vite.config.js — relative import of the vendored source (../../../../js/src/shiny-react/index) and server.fs.allow
• CLAUDE.md — repo-structure section, commands, vendoring note (js/src/shiny-react/), JS test path (js/src/shiny-react/__tests__/)
• CI workflows / pre-commit config referencing js/ paths
• Docs that reference the path: DESIGN.md, docs/posit-conf-2026-goals.md, decisions/2026-08-17-js-distribution.md, .claude/references/* if applicable
• Grep for remaining js/ path references (js/dist, js/src) after the move

Notes

• The npm package name is independent of the directory name; this rename is for repo consistency only.
• The conf-2026 goals doc plans to eventually move the vendored hooks to their own repository, so keep the rename purely mechanical.

Refs #172, decisions/2026-08-17-js-distribution.md.