feat: right-size schema-consistency-checker with max-turns cap and pre-computed schema diff step

- Add max-turns: 60 to engine config (was unconstrained, causing 138-turn runs)
- Add checkout: fetch-depth: 1, current: true for clean repo state
- Add pre-agent-steps with deterministic bash script that pre-computes:
  - All schema top-level fields (jq)
  - All yaml-tagged struct fields from pkg/parser and pkg/workflow (grep)
  - All frontmatter keys used in .github/workflows/*.md
  - Schema field types
  - Field gap diffs (in_schema_not_parser, in_parser_not_schema, etc.)
  - Writes result to /tmp/gh-aw/agent/schema-diff.json
- Update Implementation Steps to instruct agent to start from pre-computed data
- Update Efficiency guidelines to reinforce use of pre-computed data

Fixes: #28688 (deep-report right-size schema consistency checker)"

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/db6101c0-1700-4f5b-a09d-e116df370705

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

Author: Copilot