BACK-471 - Display task dates consistently as UTC in TUI, plain output, and MCP

[MrLesk]

Summary

• Add a shared UTC display formatter for Backlog date fields.
• Append (UTC) to CLI plain task/comment dates and MCP task/document date text.
• Route TUI task detail comment dates through the same UTC formatter without changing stored markdown values or Web local-time display.

Verification

• bunx tsc --noEmit
• bun run check .
• bun test src/test/utc-date-display.test.ts src/test/cli-plain-output.test.ts src/test/mcp-tasks.test.ts src/test/mcp-documents.test.ts

[MrLesk]

@codex review pls

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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