refactor: remove Go envd module, update host agent for Rust envd

The Go envd guest agent (`envd/`) is fully replaced by the Rust
implementation (`envd-rs/`). This commit removes the Go module and
updates all references across the codebase.

Makefile: remove ENVD_DIR, VERSION_ENVD, build-envd-go, dev-envd-go,
and Go envd from proto/fmt/vet/tidy/clean targets. Add static-link
verification to build-envd.

Host agent: rewrite snapshot quiesce comments that referenced Go GC
and page allocator corruption — no longer applicable with Rust envd.
Tighten envdclient to expect HTTP 200 (not 204) from health and file
upload endpoints, and require JSON version response from FetchVersion.

Remove NOTICE (no e2b-derived code remains). Update CLAUDE.md and
README.md to reflect Rust envd architecture.

envd-rs/README.md

@@ -103,7 +103,7 @@ src/
 ├── state.rs             # Shared AppState
 ├── config.rs            # Constants
 ├── conntracker.rs       # TCP connection tracking for snapshot/restore
-├─<EFBFBD><EFBFBD><EFBFBD> execcontext.rs       # Default user/workdir/env
+├── execcontext.rs       # Default user/workdir/env
 ├── logging.rs           # tracing-subscriber (JSON or pretty)
 ├── util.rs              # AtomicMax
 ├── auth/                # Token, signing, middleware