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>

docs/tech/architecture.md

@@ -409,7 +409,7 @@ Intended for local development; CI uses the parallel matrix instead.
 ### `molecule/discover_runners.py`
 
 Discovers available Gitea Actions runners for molecule tests. This is the
-canonical implementation; `devx.ci.discover_runners` is a deprecated wrapper
+canonical implementation (formerly `devx.ci.discover_runners`, removed in v0.51.0)
 that re-exports from this module. Queries runners at repository,
 organization, and instance (administrator) levels, with warnings logged
 to stderr on non-200 responses (except 403 on instance-level, which is

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`