feat(envd): update guest agent for Cloud Hypervisor

Remove Firecracker-specific MMDS metadata fetching and metrics host
module. CH communicates with the guest purely over TAP networking,
so MMDS (Firecracker's metadata service via MMDS address) is no longer
needed.

- Remove src/host/ module (mmds.rs, metrics.rs)
- Remove reqwest dependency (was only used for MMDS HTTP calls)
- Remove --isnotfc CLI flag (no longer dual-mode)
- Simplify health endpoint and init handler
- Update state management for CH snapshot lifecycle
- Bump version to 0.3.0

envd-rs/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "envd"
-version = "0.2.1"
+version = "0.3.0"
 edition = "2024"
 rust-version = "1.88"
 
@@ -53,9 +53,6 @@ notify = "7"
 # Compression
 flate2 = "1"
 
-# HTTP client (MMDS polling)
-reqwest = { version = "0.12", default-features = false, features = ["json"] }
-
 # Directory walking
 walkdir = "2"