feat(evaluator): make skill-trigger target-agnostic

[christso]

Objective

Make the skill-trigger evaluator work across all agent providers (pi, copilot, codex, etc.), not just Claude Code.

Currently, skill-trigger (in packages/core/src/evaluation/evaluators/skill-trigger.ts) hardcodes detection for two Claude Code-specific tool names:

• Skill tool — checks input.skill for skill name substring
• Read tool — checks input.file_path for skill name substring

Any other provider (pi, copilot, codex) never emits these tool names, so skill-trigger always fails with "No tool calls recorded" or "First tool was X — not Skill/Read".

Architecture Boundary

core-runtime — this is a built-in evaluator in packages/core/src/evaluation/evaluators/.

Design Latitude

Implementation approach is open. Some options to consider:

1. Provider-specific tool name mappings — each provider declares which tool names correspond to "invoke skill" and "read file" semantics, and the evaluator checks against the active provider's mapping.
2. Configurable tool matchers in EVAL.yaml — let the eval author specify which tool name + input field to match (e.g., tool: "Skill", input_field: "skill" as defaults, overridable per-provider or per-test).
3. Generic first-tool-call pattern matcher — generalize beyond skill detection to "did the first tool call match pattern X", making skill-trigger a special case of a broader tool-match evaluator.

The solution should preserve backward compatibility with existing skill-trigger assertions that assume Claude Code semantics.

Acceptance Signals

• skill-trigger assertions pass when run against a pi or copilot target that invokes the equivalent skill/file-reading action
• Existing Claude Code skill-trigger evals continue to work without modification
• examples/features/agent-skills-evals/ demonstrates cross-provider skill trigger detection

Non-Goals

• Changing how providers emit tool calls (that's provider-side, not evaluator-side)
• Adding new provider implementations
• Modifying the OTEL trace export pipeline