fix(profile): preserve harness across matrix expansion

[drewstone]

What changed

• write each selected harness to the canonical top-level AgentProfile field
• keep matrix metadata for grouping and identity
• prove base harnesses are overridden and exact harnesses reach dispatch

Why

Matrix expansion previously changed only metadata, so execution still used the base harness or no harness while reports claimed a different arm.

Verification

• 4,384 tests passed; 3 skipped
• typecheck and example typecheck passed
• build and package verification passed
• Biome and git diff checks passed
• downstream agent-runtime full suite passed: 1,771 tests; 5 skipped