Use agent description as prompt placeholder text

[JosXa]

Problem

Currently, the prompt input placeholder always shows randomized hint text like "Ask anything... 'debug the login flow'", regardless of which agent is active. This means users don't get contextual information about what the current agent is designed to do.

When switching between specialized agents (e.g., from build to docs to frontend-ui-ux-engineer), the placeholder text doesn't adapt to reflect the agent's purpose, missing an opportunity to guide users on what kind of tasks to request.

Proposed Solution

Display the agent's description as the placeholder text when available, falling back to randomized hints only for:

• The build agent (default mode for newcomers)
• Agents without descriptions

Benefits

1. Contextual guidance: Users immediately see what the active agent specializes in
2. Better discoverability: Agent capabilities are surfaced without needing to check documentation
3. Smoother onboarding: New users still get the polished randomized hints on the default build agent

Implementation Details

• Use agent description as placeholder when available
• Stop hint rotation when showing descriptions (avoid distraction)
• Continue rotation for build agent and agents without descriptions
• Apply to both CLI/TUI and web app

Example

When switching to the docs agent:

• Before: "Ask anything... 'refactor the authentication'"
• After: "Writes and maintains comprehensive technical documentation"

Additional Context

This enhances the agent switching UX by providing immediate feedback about what changed and what the new agent can do.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [Bug] Primary agents lack identity awareness when user switches agents - cannot determine which agent is active #7492: Primary agents lack identity awareness when user switches agents - cannot determine which agent is active
• agent description used in /agent command are cutoff #4825: agent description used in /agent command are cutoff
• [FEATURE]: Improve Agent Navigation: Tree View, Alphabetical Sorting, and Active/Inactive Filtering #6335: Improve Agent Navigation: Tree View, Alphabetical Sorting, and Active/Inactive Filtering
• [FEATURE]: indicator for the currently active subagent #6101: indicator for the currently active subagent

Feel free to ignore if none of these address your specific case.