chore(release): stamp v0.50.270 — bootstrap launcher import validation (#1315)

- CHANGELOG.md: v0.50.270 entry detailing #1315 + maintainer follow-ups
- ROADMAP.md: bump to v0.50.270, 3849 tests collected
- TESTING.md: bump header + total to 3849
- bootstrap.py: Opus advisor optional-followup — PYTHONPATH prepend comment

#1315 by @ccqqlo (113 LOC): bootstrap.py validates launcher Python can
import both yaml and run_agent.AIAgent. Companion fix to v0.50.269's #1478
— addresses the start-healthy-then-cryptic-fail mode (different from #1478's
supervisor-respawn loop).

3849 tests pass. Opus advisor verdict: ship as-is. CI green on contributor
branch + on local stage. QA harness all green.

bootstrap.py

@@ -128,6 +128,11 @@ def _python_can_run_webui_and_agent(python_exe: str, agent_dir: Path | None = No
     script = "import yaml\nfrom run_agent import AIAgent\n"
     env = os.environ.copy()
     if agent_dir:
+        # PREPEND agent_dir to PYTHONPATH so an `agent_dir/run_agent.py` wins
+        # over any stale `run_agent` package in system site-packages (sys.path
+        # order: script-dir → PYTHONPATH entries → site-packages). The
+        # "if PYTHONPATH unset" branch avoids a leading os.pathsep, which
+        # CPython would interpret as "current directory" — a footgun.
         env["PYTHONPATH"] = (
             str(agent_dir)
             if not env.get("PYTHONPATH")