refactor: add thiserror error tree and tracing logging

SunbeamError enum with typed variants (Kube, Config, Network, Secrets,
Build, Identity, ExternalTool, Io, Json, Yaml, Other) each mapping to
a process exit code. ResultExt trait replaces anyhow's .context().

main.rs initializes tracing-subscriber with RUST_LOG env filter and
routes all errors to exit codes via SunbeamError::exit_code().

Removes anyhow dependency.

Cargo.toml

@@ -6,12 +6,14 @@ description = "Sunbeam local dev stack manager"
 
 [dependencies]
 # Core
-anyhow = "1"
+thiserror = "2"
 tokio = { version = "1", features = ["full"] }
 clap = { version = "4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 serde_yaml = "0.9"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
 # Kubernetes
 kube = { version = "0.99", features = ["client", "runtime", "derive", "ws"] }