docs: fix channel agent prerequisites - #363

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

src/__tests__/commands/capacity/command.test.ts:16:24: Unexpected control character

src/services/install/mcp/CodexMcpGenerator.ts:138:9: Variable 'blockBody' is declared but never used.

src/__tests__/commands/skill.test.ts:92:5: Unexpected control character

src/commands/channel.ts:235:22: Unsafe usage of optional chaining

src/commands/channel.ts:212:20: Catch parameter 'error' is caught but never used. Unused caught errors should match /^ignore/.

src/__tests__/tui/console/PreviewActivityIndicator.test.tsx:61:35: Unexpected control character

src/__tests__/tui/console/PreviewActivityIndicator.test.tsx:45:39: Unexpected control character

src/__tests__/services/agent/resolve-agent.service.test.ts:20:33: Parameter '_agents' is declared but never used.

src/__tests__/services/agent/resolve-agent.service.test.ts:20:20: Parameter '_id' is declared but never used.