refactor: remove deprecated devx.ci.discover_runners wrapper

All workflows now use devx.molecule.discover_runners directly. The
deprecated wrapper (added in Phase 1c for backward compatibility) is
no longer referenced by any workflow in devx, grm, or infra.

- Delete src/devx/ci/discover_runners.py
- Remove `devx ci discover-runners` CLI subcommand
- Update tests to import from devx.molecule.discover_runners
- Remove TestDeprecationWrapper class
- Update docs, README, AGENTS.md, ADR-0002

DEVX-160

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 All notable changes to this project will be documented in this file.
 
+## [Unreleased]
+
+### Breaking Changes
+
+- Remove deprecated `devx.ci.discover_runners` wrapper. All workflows
+  now use `devx.molecule.discover_runners` directly. The `devx ci
+  discover-runners` CLI subcommand has also been removed.
+
 ## [0.50.6] - 2026-08-12
 
 ### Bug Fixes