ci: migrate pytest + Docker to PHCC (retire HA core-checkout harness) (#3)

* openspec: propose migrate-ci-to-phcc (proposal + design)

Design-first per repo convention. Migrates pytest + Docker CI off the dead HA core-checkout harness to the PHCC-based local test suite that already passes locally (164). Implementation (tasks + code) held pending review of the design's open questions.

* ci: migrate pytest + Docker off HA core-checkout to PHCC

Implements openspec change migrate-ci-to-phcc.

- pytest.yaml: run the local PHCC suite (latest + dev matrix; dev is
  continue-on-error). Fixes the trigger (was push:[master] -> never ran on
  main; now main). Drops coverage (was computed but never uploaded). Recipe
  verified in a clean env locally: 164 passed, Python 3.13, latest PHCC.
- Delete dead core-checkout scaffolding: install_dependencies action,
  setup-dependencies, setup-symlinks, update-test-matrix.{py,yaml},
  test_dependencies.py, Dockerfile, docker-build.yml.
- setup-devcontainer: PHCC flow (uv sync --group test).
- tests/README.md: rewritten for 'uv run pytest'.
- CLAUDE.md: drop the 'Refresh test matrix' row.
- hacs.json 2024.12.0 -> 2026.1.0; README min-version line corrected (it
  wrongly claimed '2025.1.0 pinned in manifest.json' - manifest pins none).

* openspec: mark migrate-ci-to-phcc 4.3 done (CI verified green)

scripts/setup-devcontainer

@@ -2,20 +2,8 @@
 set -ex
 cd "$(dirname "$0")/.."
 
-# Clone only if the folder doesn't exist
-if [[ ! -d "core" ]]; then
-    git clone --depth 1 --branch dev https://github.com/home-assistant/core.git
-fi
+# Tests run via pytest-homeassistant-custom-component (PHCC); no Home Assistant
+# core checkout is required. See tests/README.md.
+uv sync --group test
 
-pip install \
-    colorlog \
-    pip \
-    ruff \
-    uv
-
-pip cache purge
-
-uv venv --clear --python 3.14.2
-./scripts/setup-dependencies
-./scripts/setup-symlinks
 uv run pre-commit install-hooks