feat(m8): MCP client, tool replaceability, and plugin system

Complete the remaining M8 extensibility deliverables:

- MCP client with JSON-RPC 2.0 over stdio transport, protocol
  lifecycle (initialize/tools-list/tools-call), and process group
  management for clean shutdown
- MCP tool adapter implementing tool.Tool with mcp__{server}__{tool}
  naming convention and replace_default for swapping built-in tools
- MCP manager for multi-server orchestration with parallel startup,
  tool discovery, and registry integration
- Plugin system with plugin.json manifest (name/version/capabilities),
  directory-based discovery (global + project scopes with precedence),
  loader that merges skills/hooks/MCP configs into existing registries,
  and install/uninstall/list lifecycle manager
- Config additions: MCPServerConfig, PluginsSection with opt-in/opt-out
  enabled/disabled resolution
- TUI /plugins command for listing installed plugins
- 54 tests across internal/mcp and internal/plugin packages

docs/essentials/milestones.md

@@ -175,10 +175,10 @@ depends_on: [vision]
 - [x] Hook command types: command (shell), prompt (LLM), agent (spawn elf)
 - [x] Skill loading from .gnoma/skills/, ~/.config/gnoma/skills/, bundled, plugins
 - [x] Skill frontmatter: YAML (name, description, whenToUse, allowedTools, paths)
-- [ ] MCP client: JSON-RPC over stdio, tool discovery
-- [ ] MCP tool naming: `mcp__{server}__{tool}`
-- [ ] MCP tool replaceability: `replace_default` config swaps built-in tools
-- [ ] Plugin system: plugin.json manifest, install/enable/disable lifecycle
+- [x] MCP client: JSON-RPC over stdio, tool discovery
+- [x] MCP tool naming: `mcp__{server}__{tool}`
+- [x] MCP tool replaceability: `replace_default` config swaps built-in tools
+- [x] Plugin system: plugin.json manifest, install/enable/disable lifecycle
 - [x] `/batch` skill: decompose work into N units, spawn all via `spawn_elfs`, track progress (CC-inspired)
 - [x] Coordinator mode prompt: fan-out guidance for parallel elf dispatch, concurrency rules (read vs write)