feat(coder-labs): add Omnigent multi-agent server module

[matifali]

Summary

Adds the Omnigent registry surface:

• registry/coder-labs/modules/omnigent/ — installs Omnigent, starts the server, registers the workspace host, and exposes the Omnigent Coder app.
• registry/coder-labs/templates/omnigent-workspace/ — Docker example workspace with Omnigent, Claude Code, Codex, Coder AI Gateway, and a shallow clone of github.com/coder/coder.
• .icons/omnigent.svg — temporary placeholder icon until the official Omnigent SVG is available.

Changes

• Install uv from the Omnigent module so fresh Ubuntu images work without pre-baked dependencies.
• Run omnigent host --server "http://localhost:${port}" in the background so Coder startup reaches ready and the Coder app server has an online host for filesystem calls.
• Trust the derived Coder app origins through OMNIGENT_WS_ALLOWED_ORIGINS so browser requests through Coder Apps pass Omnigent's CSRF origin checks.
• Pass OPENAI_CODER_AIGATEWAY_SESSION_TOKEN through OMNIGENT_RUNNER_ENV_PASSTHROUGH when present so host-launched Codex runners can authenticate through Coder AI Gateway.
• Harden script rendering by base64-encoding allowed origins, quoting server flags with a Bash array, clearing stale generated agent files, preserving existing runner env passthrough values, and using safe generated agent filenames.
• Wire Claude Code and Codex through Coder AI Gateway; the template uses coder-labs/codex 5.2.1 so Codex writes env_key = "OPENAI_CODER_AIGATEWAY_SESSION_TOKEN".
• Install shared workspace tool dependencies (curl, jq, tmux, bubblewrap, Node.js 22) before Claude Code, Codex, and Omnigent setup.

Validation

• GitHub checks pass on 92a428e:
  • Validate README files
  • lint
  • Check for typos and unformatted code
  • Validate Terraform output
• Local validation in /tmp/registry-pr-925:
  • terraform fmt -check -recursive registry/coder-labs/modules/omnigent registry/coder-labs/templates/omnigent-workspace
  • terraform test in registry/coder-labs/modules/omnigent: 22 passed, 0 failed
  • Rendered install.sh.tftpl and start.sh.tftpl; bash -n passed and shellcheck --severity=warning passed
  • MODULE_CHANGED_FILES='registry/coder-labs/modules/omnigent/main.tf registry/coder-labs/modules/omnigent/scripts/start.sh.tftpl registry/coder-labs/modules/omnigent/scripts/install.sh.tftpl' ./scripts/terraform_validate.sh passed
• Full E2E in Coder workspace test-omnigent-pr-925-full using temp template version pr-925-runner-token-passthrough (a81a9875-ab14-4ed6-81e3-89e8ed298811):
  • Workspace build fix: update script path #6 succeeded and reached ready
  • GET /health returns { "status": "ok" }
  • GET /v1/hosts shows the workspace host online with Claude, Codex, native Claude, and native Codex configured
  • Host process env includes OMNIGENT_RUNNER_ENV_PASSTHROUGH and OPENAI_CODER_AIGATEWAY_SESSION_TOKEN
  • ~/.codex/config.toml uses env_key = "OPENAI_CODER_AIGATEWAY_SESSION_TOKEN"
  • Fresh Omnigent codex-native-ui session conv_caaaf498b6d74189bb2da43738b9e79b launched a Codex app-server with OPENAI_CODER_AIGATEWAY_SESSION_TOKEN in its env
  • Sending Reply with only: ok completed successfully with assistant output ok and last_task_error = null

🤖 This PR was created with the help of Coder Agents, and needs a human review. 🧑💻