fix: add file-redirection example for JSON stdin mode in mcp_cli_tools_prompt (#29649)

[Copilot]

Summary

Fixes the "automatic mcp-to-cli stdin JSON parsing" issue observed in PR #29649, where the PR body contained a raw JSON object instead of the intended markdown content.

Root Cause

When an agent tried to create a PR with a multi-field JSON payload via the safeoutputs CLI, this sequence occurred:

1. Tried pipe — printf '...' | safeoutputs create_pull_request . → ❌ blocked by Claude Code bash security ("Redirect has multiple targets")
2. Tried cat pipe — cat file | safeoutputs create_pull_request . → ❌ blocked ("multiple operations require approval")
3. Fell back to — safeoutputs create_pull_request --body - < /tmp/payload.json → ✅ allowed, but wrong: --body - reads the entire JSON object as a raw string for the body parameter, rather than parsing it and distributing title, body, branch, base to their respective fields

The correct approach — safeoutputs create_pull_request . < /tmp/payload.json — was never tried because the prompt only showed pipe examples.

Fix

Updated actions/setup/md/mcp_cli_tools_prompt.md to:

• Add a "When pipes are blocked" section showing safeoutputs create_pull_request . < /tmp/payload.json (. sentinel with < file redirection, no pipe required)
• Add an explicit warning against using --body - < file for multi-field JSON payloads — this reads the whole JSON object as the body string
• Update the "pipe from a file" example to show the correct . < file form
• Update notes to list . < file alongside printf ... | as equivalent approaches

Why . < file works

• The . sentinel tells the bridge to parse stdin as a full JSON argument object
• < file redirects stdin from a file (not a pipe — no second command)
• process.stdin.isTTY is undefined for both pipes and file redirections, so the bridge reads stdin correctly in both cases
• No bash security restrictions apply because only one executable runs

[Copilot]

Pull request overview

Updates the MCP CLI usage prompt to document a non-pipe workflow for passing multi-field JSON payloads via stdin, preventing PR bodies from accidentally becoming raw JSON when agents fall back to --body - patterns.

Changes:

• Rewords JSON-stdin guidance to emphasize supplying a JSON object via stdin (dot sentinel) rather than only “piping.”
• Adds a “When pipes are blocked” section demonstrating . < file stdin file-redirection for JSON payload mode.
• Adds an explicit warning against --body - < file for multi-field JSON payloads and updates examples/notes accordingly.

Show a summary per file

File	Description
actions/setup/md/mcp_cli_tools_prompt.md	Adds file-redirection examples and warnings so multi-field JSON stdin payloads are parsed correctly even when pipes are restricted.

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