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and page allocator corruption — no longer applicable with Rust envd.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.

README.md

@@ -8,6 +8,7 @@ Secure infrastructure for AI
 - Firecracker binary at `/usr/local/bin/firecracker`
 - PostgreSQL
 - Go 1.25+
+- Rust 1.88+ with `x86_64-unknown-linux-musl` target (`rustup target add x86_64-unknown-linux-musl`)
 - pnpm (for frontend)
 - Docker (for dev infra and rootfs builds)