Add native execution mode: run Plano without Docker

[adilhafeez]

Summary

• Adds --native flag to planoai build, up, and down commands for running Plano directly on the host without Docker
• Envoy binary auto-downloaded from tetratelabs/archive-envoy on first use, cached at ~/.plano/bin/ with version pinning
• WASM plugins and brightstaff built from source via planoai build --native
• Docker mode remains the default and is fully preserved

Sample session

$ planoai build --native
✓ WASM plugins built
✓ brightstaff built

$ planoai up config.yaml --native
✓ Configuration valid
✓ Configuration rendered
✓ Plano is running (native mode)
  http://localhost:12000

$ curl -s -X POST http://localhost:12000/v1/chat/completions \
    -H "Content-Type: application/json" \
    -d '{"model":"gpt-4o","messages":[{"role":"user","content":"Say hello in 5 words or less"}]}'
{
  "model": "gpt-4o-2024-08-06",
  "choices": [{"message": {"content": "Hi there! How are you?"}}],
  "usage": {"prompt_tokens": 15, "completion_tokens": 7, "total_tokens": 22}
}

$ planoai down --native
Plano stopped (native mode).

Files changed

File	Change
cli/planoai/native_binaries.py	New — Envoy binary download, WASM plugin and brightstaff discovery
cli/planoai/native_runner.py	New — native mode lifecycle (config rendering, double-fork process management)
cli/planoai/main.py	--native flag on build/up/down commands
cli/planoai/consts.py	Native mode constants (PLANO_HOME, ENVOY_VERSION, etc.)
cli/planoai/core.py	Fix _get_gateway_ports to support model_providers key
Dockerfile	Extract ENVOY_VERSION to ARG, pinned in sync with consts.py
docs/source/get_started/quickstart.rst	Native mode prerequisites and start instructions
docs/source/resources/deployment.rst	Native deployment section

Test plan

• planoai build --native builds WASM plugins and brightstaff
• planoai up config.yaml --native starts natively, health checks pass
• curl http://localhost:12000/v1/chat/completions routes through proxy to OpenAI
• planoai down --native cleanly stops both processes
• planoai up config.yaml (without --native) still works via Docker unchanged

[salmanap]

Before I do a line-level code review: Is the build command mandatory? Meaning if we don't build native then the developer can't run native? This presumes that they must have our source code locally.

If the idea is for us to be a stand-alone binary (much like Envoy can be) then shouldn't we support that as a first-class option. Plus I am not sure if I have seen this --native flag in other open source projects that support both a dockerized container image and a binary. Can you share examples on the developer experience that might inform me here?