fix: add post-install hint for Copilot agent path verification

[kienbui1995]

What does this PR do?

Adds a post-install warning after Copilot agent installation, reminding users to verify their VS Code chat.agentFilesLocations setting includes the install path.

Motivation

VS Code's default chat.agentFilesLocations may not include ~/.github/agents or ~/.copilot/agents, so agents installed by install.sh --tool copilot may not be detected. This is a minimal, non-invasive fix that guides users to the right setting.

Fixes #218
Relates to #185, Discussion #200

Changes

• scripts/install.sh: add warn + dim hint after install_copilot() completes

Output after install

[OK]  Copilot: 155 agents -> ~/.github/agents
[OK]  Copilot: 155 agents -> ~/.copilot/agents
[!!]  Copilot: Verify VS Code setting 'chat.agentFilesLocations' includes your install path.
         Open Settings (Ctrl/Cmd+,) -> search 'chat.agentFilesLocations'

Checklist

• Minimal change (2 lines added)
• Tested locally
• Follows existing warn/dim pattern used elsewhere in the script