Add skill body injection opt-out

[spboyer]

Trigger-precision evals need to measure whether an agent invokes a skill, but the existing skill: path injected the full target skill body into the system prompt. This adds an explicit opt-out so evals can keep their skill association while requiring the agent to use the skill tool for the body.

Summary

• Adds config.inject_skill_body, defaulting to true to preserve existing behavior.
• Threads the setting through normal eval execution and waza trigger so false suppresses only the target <skill_context> block.
• Keeps skill discovery, SDK skill directories, and compact <available_skills> summaries active unless skills are fully disabled.
• Updates schema, README, and docs site guidance for trigger-precision evals.

Testing

• go test ./...
• cd site && npm ci --silent && npm run build

Notes

This task was flagged as "needs review" because it touches medium-complexity CLI/execution behavior. Please have a squad member review before merging.

Fixes: #285

[Copilot]

Pull request overview

Adds a new evaluation config flag to opt out of injecting the full target skill/agent body into the system prompt, enabling “trigger-precision” evals to measure actual skill tool invocation while preserving skill association and discovery behavior.

Changes:

• Introduces config.inject_skill_body (default true) and threads it through eval orchestration and waza trigger via ExecutionRequest.SuppressSkillBody.
• Updates Copilot SDK execution prompt construction to conditionally omit the target <skill_context> while keeping <available_skills> summaries.
• Updates JSON schema + docs, and adds/extends unit tests across schema validation, spec parsing, orchestration/trigger runners, and skill message building.

Show a summary per file

File	Description
site/src/content/docs/reference/schema.mdx	Documents the new inject_skill_body config field and its trigger-precision use case.
site/src/content/docs/guides/eval-yaml.mdx	Adds inject_skill_body to config examples/table and documents trigger-precision guidance.
schemas/eval.schema.json	Extends eval JSON schema to allow config.inject_skill_body with default true.
README.md	Documents how to disable target skill body injection for trigger-precision evals.
internal/validation/schema_test.go	Adds schema validation coverage for inject_skill_body.
internal/trigger/runner.go	Propagates config into execution via SuppressSkillBody.
internal/trigger/runner_test.go	Verifies SuppressSkillBody is set when opting out.
internal/orchestration/runner.go	Propagates config into execution requests via SuppressSkillBody.
internal/orchestration/runner_test.go	Adds request-building coverage for default vs suppressed injection.
internal/models/spec.go	Adds InjectSkillBody *bool and ShouldInjectSkillBody() defaulting logic.
internal/models/spec_test.go	Tests defaulting + YAML deserialization of inject_skill_body.
internal/execution/skill_injection_test.go	Adds unit test ensuring target <skill_context> is omitted when suppressed.
internal/execution/engine.go	Extends ExecutionRequest with SuppressSkillBody.
internal/execution/copilot.go	Threads suppression into buildSkillSystemMessage(..., injectSkillBody) behavior.
internal/execution/copilot_test.go	Updates tests to match the new buildSkillSystemMessage signature.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 0