feat(story-81): agent roster badges show availability state with green idle styling

- Add data-testid="roster-dot-{name}" to both active and idle dot spans for testability
- Change idle badge from grey (#888, #555, #333) to green (#3fb950, #3fb95015, #3fb95040)
- Update idle label from "idle" to "available" to reinforce positive availability signal
- Update tooltip from "— idle" to "— available" for consistency
- Active/running agents retain blue (#58a6ff) pulsing dot styling unchanged
- Add 4 new Vitest tests covering green idle dot, green badge styling, blue active dot, and blue active badge

Closes story 81: Agent roster badges show availability state

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AgentPanel.test.tsx

@@ -374,3 +374,74 @@ describe("AgentPanel fade-out", () => {
 		});
 	});
 });
+
+describe("RosterBadge availability state", () => {
+	beforeAll(() => {
+		Element.prototype.scrollIntoView = vi.fn();
+	});
+
+	beforeEach(() => {
+		mockedAgents.getAgentConfig.mockResolvedValue(ROSTER);
+		mockedAgents.listAgents.mockResolvedValue([]);
+	});
+
+	it("shows a green dot for an idle agent", async () => {
+		render(<AgentPanel />);
+
+		const dot = await screen.findByTestId("roster-dot-coder-1");
+		// JSDOM normalizes #3fb950 to rgb(63, 185, 80)
+		expect(dot.style.background).toBe("rgb(63, 185, 80)");
+		expect(dot.style.animation).toBe("");
+	});
+
+	it("shows green badge styling for an idle agent", async () => {
+		render(<AgentPanel />);
+
+		const badge = await screen.findByTestId("roster-badge-coder-1");
+		// JSDOM normalizes #3fb95015 to rgba(63, 185, 80, 0.082) and #3fb950 to rgb(63, 185, 80)
+		expect(badge.style.background).toBe("rgba(63, 185, 80, 0.082)");
+		expect(badge.style.color).toBe("rgb(63, 185, 80)");
+	});
+
+	it("shows a blue pulsing dot for an active agent", async () => {
+		const agentList: AgentInfo[] = [
+			{
+				story_id: "81_active",
+				agent_name: "coder-1",
+				status: "running",
+				session_id: null,
+				worktree_path: null,
+				base_branch: null,
+			},
+		];
+		mockedAgents.listAgents.mockResolvedValue(agentList);
+
+		render(<AgentPanel />);
+
+		const dot = await screen.findByTestId("roster-dot-coder-1");
+		// JSDOM normalizes #58a6ff to rgb(88, 166, 255)
+		expect(dot.style.background).toBe("rgb(88, 166, 255)");
+		expect(dot.style.animation).toBe("pulse 1.5s infinite");
+	});
+
+	it("shows blue badge styling for an active agent", async () => {
+		const agentList: AgentInfo[] = [
+			{
+				story_id: "81_active",
+				agent_name: "coder-1",
+				status: "running",
+				session_id: null,
+				worktree_path: null,
+				base_branch: null,
+			},
+		];
+		mockedAgents.listAgents.mockResolvedValue(agentList);
+
+		render(<AgentPanel />);
+
+		const badge = await screen.findByTestId("roster-badge-coder-1");
+		// JSDOM normalizes #58a6ff18 to rgba(88, 166, 255, 0.094) and #58a6ff to rgb(88, 166, 255)
+		expect(badge.style.background).toBe("rgba(88, 166, 255, 0.094)");
+		expect(badge.style.color).toBe("rgb(88, 166, 255)");
+	});
+});