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

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

docs/tech/ci-cd-workflow.md

@@ -456,7 +456,7 @@ instance levels. Falls back to `MOLECULE_RUNNERS` repo variable or
 `DEFAULT_MAX_RUNNERS` (3).
 
 ```bash
-python -m devx.ci.discover_runners --owner <owner> --repo <repo> [--count] [--indices]
+python -m devx.molecule.discover_runners --owner <owner> --repo <repo> [--count] [--indices]
 ```
 
 ### `detect_release_commit.py`