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>

AGENTS.md

@@ -25,7 +25,7 @@ Mosaic Stack is a self-hosted, multi-user AI agent platform. TypeScript monorepo
 | `packages/brain`   | Data layer (PG-backed)          | @mosaicstack/db                  |
 | `packages/queue`   | Valkey task queue + MCP         | ioredis                          |
 | `packages/coord`   | Mission coordination            | @mosaicstack/queue               |
-| `packages/cli`     | Unified CLI + Pi TUI            | Ink, Pi SDK                      |
+| `packages/mosaic`  | Unified `mosaic` CLI + TUI      | Ink, Pi SDK, commander           |
 | `plugins/discord`  | Discord channel plugin          | discord.js                       |
 | `plugins/telegram` | Telegram channel plugin         | Telegraf                         |