feat: add panda build command for Docker image builds

[qu0b]

Summary

• Adds panda build <client> CLI command for triggering Docker image builds in eth-client-docker-image-builder
• Routes through the proxy so the GitHub token is centralized (no token needed on the client machine)
• --wait flag blocks until the build completes — designed for agent workflows
• Three-layer implementation: proxy handler -> server endpoint -> CLI command

Usage

# Trigger and return immediately
panda build geth
panda build geth --ref master
panda build lighthouse --ref unstable

# Trigger and wait for completion (agent workflows)
panda build geth --ref master --wait
panda build geth --ref master --wait --json

# Overrides
panda build geth --repo ethereum/go-ethereum --ref my-branch
panda build geth --tag my-custom-tag

Architecture

panda build geth --ref master
  -> POST server /api/v1/build/trigger
    -> POST proxy /github/actions/trigger
      -> POST github.com/.../actions/workflows/build-push-geth.yml/dispatches

panda build geth --wait  (polls until done)
  -> POST server /api/v1/build/status
    -> POST proxy /github/actions/run-status
      -> GET github.com/.../actions/runs/{id}

Proxy config

Requires a GitHub token with actions:write on the target repo:

github:
  token: "${GITHUB_TOKEN}"

Files changed

File	Change
pkg/proxy/handlers/github.go	New — GitHub Actions trigger + run-status handlers
pkg/proxy/server_config.go	Add GitHubAPIConfig struct
pkg/proxy/server.go	Wire GitHub handler + routes
pkg/server/api.go	Add build trigger + status endpoints
pkg/serverapi/types.go	Add build request/response types
pkg/cli/build.go	New — panda build command with --wait
pkg/cli/server_helpers.go	Add triggerBuild() + getBuildStatus() helpers

Test plan

• go build ./... compiles
• go test ./... passes
• golangci-lint clean on changed packages
• Manual: configure proxy with github.token, run panda build geth --ref master, verify workflow dispatched
• Manual: run panda build geth --ref master --wait, verify it polls and returns on completion