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

internal/plugin/manifest_test.go

@@ -0,0 +1,207 @@
+package plugin
+
+import (
+	"testing"
+)
+
+func TestParseManifest_Valid(t *testing.T) {
+	data := []byte(`{
+		"name": "git-tools",
+		"version": "1.0.0",
+		"description": "Git integration for gnoma",
+		"author": "vikingowl",
+		"capabilities": {
+			"skills": ["skills/*.md"],
+			"hooks": [
+				{
+					"name": "lint-before-commit",
+					"event": "pre_tool_use",
+					"type": "command",
+					"exec": "scripts/lint.sh",
+					"tool_pattern": "bash*"
+				}
+			],
+			"mcp_servers": [
+				{
+					"name": "git",
+					"command": "bin/mcp-git",
+					"args": ["--verbose"]
+				}
+			]
+		}
+	}`)
+
+	m, err := ParseManifest(data)
+	if err != nil {
+		t.Fatalf("ParseManifest: %v", err)
+	}
+
+	if m.Name != "git-tools" {
+		t.Errorf("Name = %q, want %q", m.Name, "git-tools")
+	}
+	if m.Version != "1.0.0" {
+		t.Errorf("Version = %q, want %q", m.Version, "1.0.0")
+	}
+	if len(m.Capabilities.Skills) != 1 {
+		t.Errorf("Skills count = %d, want 1", len(m.Capabilities.Skills))
+	}
+	if len(m.Capabilities.Hooks) != 1 {
+		t.Errorf("Hooks count = %d, want 1", len(m.Capabilities.Hooks))
+	}
+	if len(m.Capabilities.MCPServers) != 1 {
+		t.Errorf("MCPServers count = %d, want 1", len(m.Capabilities.MCPServers))
+	}
+}
+
+func TestParseManifest_Minimal(t *testing.T) {
+	data := []byte(`{"name": "minimal", "version": "0.1.0"}`)
+	m, err := ParseManifest(data)
+	if err != nil {
+		t.Fatalf("ParseManifest: %v", err)
+	}
+	if m.Name != "minimal" {
+		t.Errorf("Name = %q", m.Name)
+	}
+}
+
+func TestParseManifest_InvalidJSON(t *testing.T) {
+	_, err := ParseManifest([]byte(`not json`))
+	if err == nil {
+		t.Error("expected error for invalid JSON")
+	}
+}
+
+func TestManifest_Validate(t *testing.T) {
+	tests := []struct {
+		name    string
+		m       Manifest
+		wantErr bool
+	}{
+		{
+			name:    "valid",
+			m:       Manifest{Name: "my-plugin", Version: "1.0.0"},
+			wantErr: false,
+		},
+		{
+			name:    "empty name",
+			m:       Manifest{Version: "1.0.0"},
+			wantErr: true,
+		},
+		{
+			name:    "invalid name uppercase",
+			m:       Manifest{Name: "MyPlugin", Version: "1.0.0"},
+			wantErr: true,
+		},
+		{
+			name:    "invalid name starts with number",
+			m:       Manifest{Name: "1plugin", Version: "1.0.0"},
+			wantErr: true,
+		},
+		{
+			name:    "empty version",
+			m:       Manifest{Name: "my-plugin"},
+			wantErr: true,
+		},
+		{
+			name:    "invalid version",
+			m:       Manifest{Name: "my-plugin", Version: "not-semver"},
+			wantErr: true,
+		},
+		{
+			name: "skill glob path traversal",
+			m: Manifest{
+				Name:    "bad",
+				Version: "1.0.0",
+				Capabilities: Capabilities{
+					Skills: []string{"../../../etc/passwd"},
+				},
+			},
+			wantErr: true,
+		},
+		{
+			name: "skill glob absolute path",
+			m: Manifest{
+				Name:    "bad",
+				Version: "1.0.0",
+				Capabilities: Capabilities{
+					Skills: []string{"/etc/passwd"},
+				},
+			},
+			wantErr: true,
+		},
+		{
+			name: "hook exec path traversal",
+			m: Manifest{
+				Name:    "bad",
+				Version: "1.0.0",
+				Capabilities: Capabilities{
+					Hooks: []HookSpec{{
+						Name:  "h",
+						Event: "pre_tool_use",
+						Type:  "command",
+						Exec:  "../../../bin/evil",
+					}},
+				},
+			},
+			wantErr: true,
+		},
+		{
+			name: "mcp command path traversal",
+			m: Manifest{
+				Name:    "bad",
+				Version: "1.0.0",
+				Capabilities: Capabilities{
+					MCPServers: []MCPServerSpec{{
+						Name:    "evil",
+						Command: "../../../bin/evil",
+					}},
+				},
+			},
+			wantErr: true,
+		},
+		{
+			name:    "valid name with hyphens and numbers",
+			m:       Manifest{Name: "my-plugin-2", Version: "0.1.0"},
+			wantErr: false,
+		},
+		{
+			name:    "valid name with underscores",
+			m:       Manifest{Name: "my_plugin", Version: "0.1.0"},
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.m.Validate()
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Validate() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestValidSemver(t *testing.T) {
+	tests := []struct {
+		v    string
+		want bool
+	}{
+		{"1.0.0", true},
+		{"0.1.0", true},
+		{"12.34.56", true},
+		{"1.0", false},
+		{"1", false},
+		{"v1.0.0", false},
+		{"1.0.0-beta", false}, // strict semver only for v1
+		{"", false},
+		{"not-a-version", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.v, func(t *testing.T) {
+			if got := validSemver(tt.v); got != tt.want {
+				t.Errorf("validSemver(%q) = %v, want %v", tt.v, got, tt.want)
+			}
+		})
+	}
+}