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>

pnpm-workspace.yaml

@@ -2,7 +2,6 @@ packages:
   - 'apps/*'
   - 'packages/*'
   - 'plugins/*'
-  - '!packages/cli' # merged into @mosaicstack/mosaic
 
 ignoredBuiltDependencies:
   - '@nestjs/core'