Add sigops.notify_slack tool, fix clippy warnings - #2

…covery, task execution

Complete implementation of the SigOps infrastructure agent in Rust:

- config.rs: CLI args + env vars via clap (server URL, agent ID, tenant, token, interval)
- discovery.rs: Host info collection + tool discovery (systemctl, docker, kubectl, curl, etc.)
- heartbeat.rs: HTTP heartbeat client with JSON payload to SigOps server
- executor.rs: Task execution engine with 4 built-in tools:
  - sigops.restart: Service restart with input sanitization
  - sigops.http: HTTP requests via curl
  - sigops.condition: Numeric expression evaluation
  - sigops.wait: Timed delay with 3600s max
- main.rs: Tokio async runtime, heartbeat loop, tracing logging

Infrastructure:
- GitHub Actions CI (Rust toolchain + cache + build + test + clippy)
- 18 unit tests, all passing
- Cargo.lock committed for reproducible builds

Dependencies: tokio, reqwest, serde, clap, tracing, uuid, hostname, thiserror

https://claude.ai/code/session_016BNDbB5ERJFsXQZiuL6tL1