fix: prevent UTF-8 char boundary panics in agent handlers

[darioandyoshi-tech]

This PR fixes multiple panics caused by unsafe byte-slicing of strings that may contain multi-byte UTF-8 characters.

The fix replaces byte-based slicing with a UTF-8 aware approach using .chars().take().collect() to ensure truncation occurs on valid character boundaries.

Fixes:

• agent_cmd/handlers/list.rs
• agent_cmd/handlers/show.rs
• agent_cmd/handlers/generate.rs

Reference: PlatformNetwork/bounty-challenge#3920

[greptile-apps]

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