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/decisions/0002-ansible-check-consolidation-and-wait-for-checks.md

@@ -81,17 +81,13 @@ Extract the inline polling logic into `devx.ci.wait_for_checks`:
 This replaces the inline shell polling in `grm` `ci.yml` with a
 reusable, testable Python module.
 
-### 3. Deprecated `ci/discover_runners` Wrapper
+### 3. Removed `ci/discover_runners` Wrapper
 
-Merge the `ci/discover_runners` implementation (with its better error
+Merged the `ci/discover_runners` implementation (with its better error
 logging) into `molecule/discover_runners` as the canonical version.
-Make `ci/discover_runners` a deprecated wrapper that:
-
-- Re-exports all public symbols from `molecule.discover_runners`
-- Emits a `DeprecationWarning` when run as `__main__`
-- Preserves backward compatibility for existing workflow references
-
-New code should import from `devx.molecule.discover_runners` directly.
+The `ci/discover_runners` wrapper was deprecated in Phase 1c and
+**removed in Phase 2d** (DEVX-160). All workflows now use
+`devx.molecule.discover_runners` directly.
 
 ## Consequences