feat: inject available agents into system prompt

[mrjasonroy]

Summary

• When no agent is selected, the system prompt now includes a list of available agents with their names and descriptions
• This allows the AI to suggest relevant agents when the user's request would benefit from specialized expertise
• Users are informed they can select agents via @mention, tools menu, or sidebar

Changes

• Added rememberAvailableAgentsAction with 5-minute cache in actions.ts
• Updated buildUserSystemPrompt to accept optional availableAgents parameter
• Added <available_agents> section to prompt when no agent is selected
• Added cache key for available agents
• Added unit tests for the new functionality

Test plan

• Types check passes
• Lint passes
• Unit tests pass (5 new tests for prompt builder)
• Manual test: Start a new chat without an agent, verify AI can suggest agents
• Manual test: Start a chat with an agent selected, verify agents section is not included

🤖 Generated with Claude Code

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[theuargb]

i think this could be very useful, possibly guarded with .env feature flag.

i think what we call Agents are really Skills now,
and skill activation should be automated for the ux.

if better-chatbot will evolve agents into skills this idea seems to be solid foundation