Add the end-to-end test, run after each milestone (M1.6)

tests/integration/test_end_to_end.py drives the whole assembled agent
(real models, the Gitea sandbox, a real build host) through a milestone
with a scripted operator, and checks what bring-up run 3 checked by hand:
a PR holding only the work, pushed with the push credential; no
credential in the clone; the planner's view carrying facts, not the
executor's turns. It cleans up after itself on the sandbox and the build
host.

scripts/e2e.sh runs it on the bring-up build VM (or --local). It has its
own marker, e2e, kept out of check.sh and CI since it spends tokens.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

scripts/check.sh

@@ -6,5 +6,5 @@ cd "$(dirname "$0")/.."
 uv sync --all-packages
 uv run ruff check .
 uv run ruff format --check .
-uv run pytest -m "not integration"
-uv run pytest -m integration
+uv run pytest -m "not integration and not e2e"
+uv run pytest -m "integration and not e2e"