Add managed agent avatars

[wesbillman]

Summary

• map known managed agent runtimes to official avatar URLs
• sync avatar URLs onto the managed agent profile during create-time profile sync
• cover command alias and path resolution with targeted discovery tests

Testing

• source ./bin/activate-hermit && cd desktop && pnpm check
• source ./bin/activate-hermit && cd desktop/src-tauri && cargo test --lib managed_agents::discovery::tests

Closes #91