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>

CLAUDE.md

@@ -10,7 +10,7 @@ Self-hosted, multi-user AI agent platform. TypeScript monorepo.
 - **Web**: Next.js 16 + React 19 (`apps/web`)
 - **ORM**: Drizzle ORM + PostgreSQL 17 + pgvector (`packages/db`)
 - **Auth**: BetterAuth (`packages/auth`)
-- **Agent**: Pi SDK (`packages/agent`, `packages/cli`)
+- **Agent**: Pi SDK (`packages/agent`, `packages/mosaic`)
 - **Queue**: Valkey 8 (`packages/queue`)
 - **Build**: pnpm workspaces + Turborepo
 - **CI**: Woodpecker CI