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>

AGENTS.md

@@ -167,7 +167,6 @@ src/devx/
 │   ├── cancel_superseded_runs.py  # Cancel in-flight CI runs for the same PR branch
 │   ├── check_workflow_artifact_deps.py  # Verify artifact download jobs depend on upload jobs
 │   ├── check_workflow_tofu_init.py  # Verify tofu-state jobs have a tofu-init step
-│   ├── discover_runners.py  # Deprecated wrapper → molecule/discover_runners
 │   └── wait_for_checks.py  # Poll Gitea Actions for job completion (replaces inline shell polling)
 ├── tools/               # Developer tooling modules (run locally or by CI)
 │   ├── setup.py         # Environment setup (venv, deps, hooks)
@@ -220,7 +219,7 @@ src/devx/
 │   ├── ui.py             # say() — unified click.echo + logging output
 │   └── jinja.py          # Jinja2 environment helpers + Ansible-compatible filters
 └── molecule/            # Optional molecule testing helpers (for Ansible projects)
-    ├── discover_runners.py    # Dynamic Gitea runner discovery (canonical; ci/discover_runners is a deprecated wrapper)
+    ├── discover_runners.py    # Dynamic Gitea runner discovery
     ├── distribute_molecule.py # Distribute molecule scenarios across runners (LPT scheduling, --roles-root for multi-role)
     ├── molecule_ci_guard.py   # Run molecule with cross-runner fail-fast (--roles-root)
     ├── molecule_all.py        # Run all molecule scenarios locally