Add first generic AI agent definition

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Summary

• Rename the agent directory to ai-agents/.
• Add the first generic agent definition at ai-agents/hello.md.
• Ask the agent to invent its own short name instead of naming it after a provider.
• Explicitly instruct the agent not to reveal secrets, tokens, or environment variables.

This PR is the foundational agent definition for the cross-repository AI agent experiment.

Related PRs

• Generic reusable workflow in CTActions: Add generic AI agent workflow CTActions#72
• CTBase manual test workflow: Add manual generic AI agent test CTBase.jl#559

Test plan

• Review the generic agent instructions.
• Exercise the agent from the linked CTBase workflow after all three branches are available.

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This PR is the first step in a three-repository experiment:

1. This PR defines the agent instructions.
2. CTActions PR #72 provides the reusable workflow that loads this agent and calls Albert.
3. CTBase PR #559 provides the manual end-to-end test entry point.

The initial test will use the development branches from all three repositories before the changes are merged.

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The design is now provider-independent at the agent-definition level. The agent is stored as ai-agents/hello.md, does not mention Albert, and invents a short name for itself when called.

The corresponding generic runner is in CTActions PR #72, and the manual test entry point is in CTBase PR #559.