[Proposal] Deprecate Python plugin framework and consolidate on Go

## Proposal: Deprecate the Python plugin framework and consolidate on Go

### Summary

Propose to deprecate the Python plugin framework (`backend/python/`, a.k.a. PyDevLake) and remove it from the codebase after a 3-month deprecation window. Going forward, all plugins should be written in Go. This reduces maintenance overhead, simplifies the Docker image and CI, and focuses contributor effort on a single, well-supported runtime.

### Background

The Python plugin framework was introduced in Feb 2023 (#3702, #4438) to lower the barrier for writing plugins. In practice it never saw adoption:

- Only **one** plugin ever shipped on it: `azuredevops` (Python).
- The `dbt` Python plugin was already removed on 2026-08-31 (#8970).
- A fully-featured Go implementation, `azuredevops_go`, now exists and is actively developed (on-premises support landed 2026-08-16, #9014).

### Evidence: Python plugins are not being maintained

| Plugin | Last meaningful commit | Note |
| --- | --- | --- |
| `azuredevops` (Python) | 2025-12-31 `feat: add environment_pattern` (#8671) | No feature/fix since; only deps refresh & runtime bumps |
| `pydevlake` framework | 2025-12-31 (same as above) | Last real fix: #8957 on 2026-07-15 (missing composite PKs) |
| `dbt` (Python) | removed 2026-08-31 (#8970) | Already gone |

In the last ~12 months, the only commits touching `backend/python/` are dependency refreshes, Poetry/uv lock bumps, a Python 3.9→3.11 runtime bump, and a single primary-key fix. Meanwhile the Go side shipped 40+ plugins and continues to receive features weekly.

### Benefits

1. **One runtime to maintain.** Eliminate the parallel PyDevLake framework, its RPC bridge, `pydevlake` package, `uv`/Poetry tooling, and Python-specific test/build scripts.
2. **Simpler Docker build.** Today `backend/Dockerfile` is based on `python:3.11-slim-bookworm`, installs python3-dev/pip/uv, copies `python/`, and runs `build.sh`. After removal the image can be a slim Go-based image — smaller, fewer CVEs, faster builds. `Dockerfile.local`'s "empty python plugins dir" workaround also goes away.
3. **Simpler CI.** Two workflows (`.github/workflows/test.yml`, `test-e2e.yml`) and Make targets (`python-dep`, `python-unit-test`, `build-python`, `build-pydevlake`) can be dropped.
4. **Clearer contributor story.** New plugin authors get one path (Go) with 40+ reference implementations instead of choosing between two frameworks where one is unmaintained.
5. **No functional loss.** The only remaining Python plugin (`azuredevops`) is superseded by `azuredevops_go`, which already has broader feature coverage (Server/on-prem support).

### Plan & Timeline

| Phase | Window | Actions |
| --- | --- | --- |
| **1. Warning** | T+0 (on issue merge) | Add deprecation notices to: `backend/python/README.md`, plugin `README.md`s, server startup log line, and `config-ui` plugin selection UI. Mark `azuredevops` (Python) as deprecated in the plugin registry; point users to `azuredevops_go`. |
| **2. Freeze** | T+1 month | Stop accepting new features/fixes for Python plugins. Only critical security fixes land. Add a CI gate rejecting new files under `backend/python/plugins/`. |
| **3. Migration assist** | T+1~T+3 months | Ship a migration guide + script to move any remaining `azuredevops` (Python) connections/scopes to `azuredevops_go`. Document data-layer equivalence. |
| **4. Removal** | T+3 months | Delete `backend/python/`, PyDevLake, the Python Dockerfile stage, `make python-*` targets, and Python CI jobs. Remove `azuredevops` (Python) from the plugin registry. |

Total deprecation window: **3 months** from the date this proposal is accepted.

### Risks & Mitigations

- **Existing users of `azuredevops` (Python):** Mitigated by `azuredevops_go` parity + migration guide in Phase 3. The Go plugin already covers cloud + on-prem.
- **External contributors with in-progress Python plugins:** The 3-month window + the warning phase gives them time to port. No private Python plugins are known to exist in the wild.
- **Reverting later:** Not needed; Go covers all current and planned use cases.

### What we need

- 👍 / 👎 from maintainers and community on deprecating Python.
- Owners to confirm `azuredevops_go` feature parity with the Python `azuredevops` plugin (esp. CI/CD environment extraction).
- Volunteers to write the Phase-3 migration guide.

Start by reviewing backend/python/, backend/Dockerfile, Dockerfile.local, .github/workflows/test.yml, .github/workflows/test-e2e.yml, and the listed Make targets. Then compare the Python azuredevops plugin with azuredevops_go, especially CI/CD environment extraction. Done means maintainers agree on the proposal and owners are identified for the warning, migration, and removal phases.

Written by the indexing model from the issue text.