Return markdown from MCP command calls #1892

Part of #1357. Plan: cli-first-mcp task 3.4 (design, tasks). Depends on #1890.

Catalog tools format stack traces, traces, and issue summaries for agents. format: "markdown" on the execute tools returns the command's plain markdown output instead of JSON, for commands with a markdown renderer.

Done when

• Tests for issue view and trace view in markdown.

via @BYK.

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