Improve post MCP installation message

[KevyVo]

Description

Currently, the CLI prints the message:
“Run defang mcp setup to install the Defang MCP server.”
This leaves users a bit confused about what to do next. Adding more context would help better guide them through the next steps.

Linked Issues

#1746

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

Summary by CodeRabbit

• Documentation
  • Enhanced post-installation instructions to guide users on using the Defang MCP server with their choice of coding agents or the built-in agent option.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The installation script gains two additional user-guidance messages displayed in interactive terminal sessions. These messages inform users about using the Defang MCP server with various coding agents and the option to run Defang's built-in agent, with no changes to control flow or error handling.

Changes

Cohort / File(s)	Summary
User Guidance Messages src/bin/install	Added two echo lines in the final user instructions block (TTY-only): clarifies MCP server compatibility with coding agents and promotes built-in agent option

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• avoid printing mcp setup message in CI #1806: Modifies the same installation script file to add/condition echo lines for interactive TTY output user guidance.

Suggested reviewers

• jordanstephens
• lionello

Poem

🐰 A rabbit hops with glee so bright,
Two helpful lines now shine in sight!
"Use any agent, or ours instead,"
The script now guides where users tread! 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: adding improved post-installation messaging to guide users on MCP server usage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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