fix(app-server): make agent file IO async in API handlers

[694410194]

Summary

• replace blocking std::fs agent file reads with async tokio::fs helpers for listing and loading agents
• switch create, update, delete, and import agent endpoints to async filesystem operations
• add focused async tests for agent file parsing and markdown filtering

Testing

• cargo test --manifest-path Cargo.toml -p cortex-app-server read_agent_file_parses_frontmatter_async
• cargo test --manifest-path Cargo.toml -p cortex-app-server list_agents_in_dir_ignores_non_markdown_files
• cargo check --manifest-path Cargo.toml -p cortex-app-server

Related bounty issue: PlatformNetwork/bounty-challenge#53256

/claim #53256

[greptile-apps]

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