feat: add slack-cli WASM package (Go → WASI build)

[airhorns]

This adds the slack-cli package for WASI for use inside the sandbox. Its a commonly-depended-on binary used by a bunch of popular skills on clawhub so I think it merits inclusion in the sandbox'd registry. It's the first go program in the registry, so I tried to copy the same approach we use for c and added a new spot for vendored go dependencies. slack-cli's transitive dependencies also annoyingly need some patches for WASI which Claude cooked up for us here.

Summary

• Adds @rivet-dev/agent-os-slack-cli, the first Go-compiled WASM package in the
  registry, providing the slack command (slackapi/slack-cli) for building Slack apps
  from inside agentOS VMs
• Introduces the Go WASM build pipeline at registry/native/go/, following the same
  pattern as the existing Rust and C pipelines: clone upstream at a pinned commit,
  vendor dependencies, apply WASI compatibility patches, and compile with GOOS=wasip1
  GOARCH=wasm
• Three WASI stub patches fix platform-incompatible dependencies: atotto/clipboard (no
  clipboard in WASM), charm.land/bubbletea (no terminal resize/suspend), and
  go-git/go-git (no Windows symlink check)

How to build

  cd registry
  make build-wasm-go   # clones slack-cli, vendors, patches, builds (~38MB WASM binary)
  make copy-wasm       # copies to software/slack-cli/wasm/

Requires Go 1.21+ (the upstream slack-cli pins Go 1.26.1).