chore: remove legacy @mosaicstack/cli package

The @mosaicstack/cli package was consolidated into @mosaicstack/mosaic in
a prior refactor, and was already excluded from pnpm-workspace.yaml. This
commit deletes the now-dead packages/cli directory and updates the last
remaining docs references that still pointed at it.

- Delete packages/cli/ (48 files, ~6.6K lines of dead code)
- pnpm-workspace.yaml: drop the !packages/cli exclusion
- tools/install.sh: update install step label
- AGENTS.md, CLAUDE.md: update package map
- README.md, docs/guides/user-guide.md, packages/mosaic/framework/defaults/README.md: update install docs

Verified clean: typecheck, lint, format:check, test (38/38 tasks).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tools/install.sh

@@ -212,7 +212,7 @@ fi
 # ═══════════════════════════════════════════════════════════════════════════════
 
 if [[ "$FLAG_CLI" == "true" ]]; then
-  step "@mosaicstack/cli (npm package)"
+  step "@mosaicstack/mosaic (npm package)"
 
   # Ensure prefix dir
   if [[ ! -d "$PREFIX" ]]; then