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>

README.md

@@ -172,7 +172,7 @@ python -m devx.ci.notify_failure --repo oblachno-oss/devx --run-id 123 \
   --workflow ci --commit abc123 --auto-login
 
 # Discover available Gitea Actions runners
-python -m devx.ci.discover_runners --owner oblachno-oss --repo devx --indices
+python -m devx.molecule.discover_runners --owner oblachno-oss --repo devx --indices
 
 # Distribute files across parallel runners (round-robin)
 python -m devx.ci.distribute_files --pattern "tests/integration/test_*.py" \