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/user/getting-started.md

@@ -142,7 +142,7 @@ infrastructure = [
 - `devx.ci.notify_failure` — Create Gitea issues on CI failures
 - `devx.ci.distribute_files` — Parallel test file distribution
 - `devx.ci.distribute_items` — Parallel item distribution across runners
-- `devx.ci.discover_runners` — Dynamic runner discovery via Gitea API
+- `devx.molecule.discover_runners` — Dynamic runner discovery via Gitea API
 
 ### Development Tools (`devx.tools.*`)