fix(core): wake parent on terminal child status

[3nesdeniz]

Problem

When an interactive child exhausts its turn budget, Strix marks it as stopped.
The parent can nevertheless remain parked in wait_for_message because only the
child's wake event is set and no inbox item is delivered to the parent.

Root cause

Terminal-state propagation was split across call sites. Crashes had a dedicated
parent notification, normal completion sent a report from agent_finish, and
stopped / failed transitions had no parent notification at all. Graceful
stops also assigned stopped directly, bypassing the coordinator's status
transition path.

Changes

• Treat completed, stopped, failed, and crashed as terminal transitions
  in AgentCoordinator.set_status.
• Deliver one high-priority terminal-status message to the actual parent on the
  first transition into a terminal state, including the failure reason when
  present.
• Keep agent_finish's structured completion report as the primary
  notification and suppress a duplicate generic message. If report delivery
  fails, make one generic fallback attempt.
• Route graceful stops through the same terminal-transition path.
• Avoid interrupting the caller's own active turn when it stops its direct
  child: the tool result already confirms that stop. If a caller stops a deeper
  descendant, notify that target's actual parent instead.
• For cascades, notify only the top-level target's external parent; internal
  parents are part of the same stop cascade.
• Remove the crash-only notification path now covered by the coordinator.
• Add regression coverage for the exact MaxTurnsExceeded path, all four
  terminal statuses, idempotence, completion-report delivery fallback,
  direct/deep stop routing, and cascade notification behavior.

Validation

• uv run pytest -q — 412 passed
• uv run pytest -q tests/test_agent_terminal_notifications.py — 14 passed
• uv run ruff check .
• uv run ruff format --check strix/core/agents.py strix/core/execution.py strix/tools/agents_graph/tools.py tests/test_agent_terminal_notifications.py
• uv run mypy strix/core/agents.py strix/core/execution.py strix/tools/agents_graph/tools.py tests/test_agent_terminal_notifications.py
• uv run pyright --pythonpath .venv/bin/python tests/test_agent_terminal_notifications.py
• uv run bandit -q -c pyproject.toml strix/core/agents.py strix/core/execution.py strix/tools/agents_graph/tools.py
• git diff --check

I also ran the documented make check-all. On a clean upstream/main
worktree at cd8270c, its repository-wide mypy step reports the same existing
70 errors (primarily Textual typing drift and missing Pygments stubs), and the
repository-wide Bandit target reports the same existing B105/B310 findings.
The changed-source and new-test checks listed above are clean.

Fixes #870

[5hy7xz92nd-oss]

Perfect and run and test all