Spike: structured CLI output (io.Writer / MCP stdio mode)

[leggetter]

Problem statement

User-visible output is scattered across fmt.Print*, Cobra, the interactive login path, and MCP. Stdio MCP requires a strict contract: stdout = JSON-RPC only; everything else should go to stderr or an injected writer. Violations regress MCP hosts (e.g. "not valid JSON" in the client). Phase 1 fixes the known gateway mcp + Execute() paths; a broader design reduces repeat risk.

Direction to explore

• A small presentation / output boundary, e.g. UserOutput with Out, Err io.Writer on config or cmd, defaulting to os.Stdout / os.Stderr.
• Modes: TTY vs MCPStdio (or NonInteractive) chosen once per invocation (command leaf, env, or context).
• Execute() and other high-touch paths use Err (or mode-aware writers) instead of hardcoded stdout for messages when in MCP stdio mode.
• Longer term: inventory and migrate direct fmt.Print where protocol safety matters.

Questions for the spike

• How much of the codebase writes non-protocol text to stdout? (grep / inventory)
• Should MCP mode be env-only, command-derived, or both?
• logrus / verbose logging: ensure debug does not hit stdout in MCP.
• Tests: subprocess gateway mcp + fake stdin; assert no non-JSON lines on stdout until JSON-RPC (may overlap Phase 1d acceptance tests).

Exit criteria

Close with a short design note + follow-up epic, or won't do with rationale (cost vs. rare footguns).