fix(multi-agent): drop best from the role model values and the frontier default #7990

Refs: #6916

Agent Brief

Type: Bug
Summary: best stops being a role model value, a frontier alias and an accepted --session-model, per the owner decision on #6916.

Current behavior:
The multi-agent config reference lists best as a role model alias and the frontier default is fable,best; the bundled defaults repeat both; resolve-roles.sh's ALIASES accepts best, which also validates --session-model; the setup skill and the drift-audit workflow's model list name it. The sub-agents docs list only sonnet, opus, haiku, fable and inherit as subagent aliases (https://code.claude.com/docs/en/sub-agents.md, choose a model).

Desired behavior:
best is accepted nowhere: a role model of best and --session-model best are rejected with the resolver's usual unknown-alias error, so a caller omits the session model and the session counts as frontier (an unknown session already does). The frontier default is fable. The config reference and route skill point at sub-agents#choose-a-model and model-config#model-aliases, with as-of dates and recheck triggers, wherever they still restate the alias list or the subagent model fallback; existing pointers stay.

Key interfaces:

• resolve-roles.sh ALIASES and its --session-model validation.
• The frontier default and roles.<role>.model values in the bundled defaults and config reference.
• The drift-audit workflow's MODELS list.

Acceptance criteria:

• git grep -n -w best -- plugins/multi-agent/reference plugins/multi-agent/scripts/resolve-roles.sh plugins/multi-agent/workflows plugins/multi-agent/skills/setup returns no model-alias use.
• scripts/run-test-file.sh plugins/multi-agent/scripts/resolve-roles.test.sh passes, including a case where --session-model best is rejected and a case where an omitted session model fans out off frontier.
• check-skill.sh passes for each edited skill.
• A multi-agent changelog fragment is added and scripts/check-changelog-fragments.sh passes.

Out of scope:

• Eval sample text that quotes a past best session.
• The session-flow orchestrate pointers and the full repo-mode sweep (sibling children).

This was generated by AI during backlog shaping. Split from #6916; the owner approved filing it in an attended backlog-shaping session on 2026-10-18.