Per-subagent `advisor` frontmatter field (pair each subagent's executor with its own advisor)

Problem

The advisor tool is currently exposed only at the session level (advisorModel setting, /advisor, --advisor), and subagents automatically inherit that single session advisor. Multi-agent orchestration frameworks assign different executor models per role via the subagent model: frontmatter field — a cheap model for mechanical agents, a stronger model for planning/review. They cannot extend that per-role tiering to advisors: every subagent is forced to share the one session-wide advisor (or none). There is no way to express, for example, "no advisor on the Haiku mapping agent, but a Fable 5 advisor on the Sonnet executor agent."

Proposed solution

Add an advisor: (and/or advisorModel:) subagent frontmatter field in .claude/agents/*.md, a sibling to the existing model: field, that overrides the inherited session advisor for that subagent — including an explicit value to opt a subagent out (no advisor). Enforce the same executor/advisor pairing rules already validated at the session level.

Why

Lets orchestration layers apply advisor tiering per role, spending advisor tokens only where a stronger reviewer pays off — matching the advisor tool's own cost guidance. The underlying primitive already exists (session-level advisor); this makes it addressable per subagent instead of all-or-nothing per session.

Current state (for reference)

• Session-level knobs today: /advisor <model>, advisorModel setting, --advisor, CLAUDE_CODE_DISABLE_ADVISOR_TOOL.
• Subagents inherit the session advisor automatically.
• Supported subagent frontmatter fields do not include any advisor field.

Downstream use case

GSD (https://github.com/open-gsd/gsd-core) assigns per-agent model tiers via model: frontmatter and a model-profile system; a per-subagent advisor field would let it pair each agent's executor with an appropriate advisor. This is the primitive an orchestration layer needs to make advisor tiering per-role instead of session-wide.

References

• Advisor tool (API): https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool
• Claude Code advisor docs: https://code.claude.com/docs/en/advisor