feat: initialize cargo workspace and project scaffold

[pentaxis93]

Summary

• initialize agentd as a Rust workspace with 5 crates
• add required project structure and stubs from issue Initialize agentd repository with Cargo workspace and project structure #1
• add technical README with status/architecture/quick-start/license sections
• verify build/test/clippy pass with clean workspace setup

Verification

• cargo fmt --all --check
• cargo build
• cargo test
• cargo clippy --workspace --all-targets --all-features -- -D warnings

Closes #1