…nstream on llm.default

Activates the M8 milestone's per-agent provider story. The framework
already resolved ``skill.model`` per-skill via
``graph.py:_build_agent_nodes -> get_llm(cfg.llm, skill.model, role=...)``;
v1.5-C uncomments the override on the example incident_management
intake skill so a default deployment (with both OLLAMA_API_KEY +
OPENROUTER_API_KEY set, or wherever ``llm.default`` resolves) shows
intake hitting Ollama Cloud's gpt-oss while the rest of the agents
follow the runtime default.

Changes:

* ``examples/incident_management/skills/intake/config.yaml`` — declare
  ``model: gpt_oss_cheap`` (was a documented but commented-out hint).
  Comment block updated to reference v1.5-C and explain the resolver.

* ``src/runtime/config.py`` — extend the ``LLMConfig.stub()`` default
  models map with stub aliases for ``gpt_oss``, ``gpt_oss_cheap``, and
  ``workhorse``. The skill-validator (``Orchestrator.create``) checks
  every ``skill.model`` against ``llm.models``; without these aliases
  the existing test suite would explode the moment intake declares
  ``model: gpt_oss_cheap`` (because tests build ``LLMConfig.stub()``
  which previously only knew ``stub_default``). The aliases route to
  the same stub provider so behaviour is unchanged for stub-mode
  callers.

* ``tests/test_per_agent_model_dispatch.py`` (new, 2 tests) — pin
  the dispatch contract:

  - ``test_build_agent_nodes_passes_skill_model_to_get_llm`` mocks
    ``runtime.graph.get_llm`` and asserts the framework calls it
    with ``model_name=skill.model`` per skill (intake gets
    ``"gpt_oss_cheap"``, triage with ``model=None`` gets ``None`` so
    ``get_llm`` falls back to ``llm.default`` downstream).
  - ``test_intake_skill_yaml_has_per_agent_override_uncommented``
    pins the YAML edit so a future refactor can't silently drop the
    override.

Live verification (``tests/test_integration_driver_s1.py`` family)
continues to require ``OLLAMA_API_KEY`` + ``OPENROUTER_API_KEY``
+ ``OLLAMA_BASE_URL`` and remains skipped without them — the human
verification gate documented at
``.planning/phases/15-real-llm-tool-loop-termination/15-VERIFICATION.md``.

Suite: 1260 passed (was 1258 — added 2), ruff clean, coverage 87.08%.