Fix CreateAgent output format for custom loader

[SlaVKsVolks]

Summary

• change CreateAgent to write custom agents as Markdown files with YAML frontmatter
• keep generated agents in .cortex/agents while matching the format loaded by CustomAgentLoader
• add regression coverage proving a newly-created project agent is discoverable by the standard loader

Bounty / Issue

• Addresses [BUG] [v0.0.7] CreateAgent tool writes .toml files into .cortex/agents, so created agents are not discoverable by the standard loader PlatformNetwork/bounty-challenge#53529

Verification

• cargo fmt --check --package cortex-engine
• cargo test -p cortex-engine test_create_agent -- --nocapture
• cargo check -p cortex-engine
• cargo build -p cortex-engine
• git diff --check

Note: cargo clippy -p cortex-engine --all-targets -- -D warnings still fails on pre-existing warnings in cortex-windows-sandbox and unrelated cortex-engine Windows sandbox/config code; this patch does not add those warnings.

[greptile-apps]

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