docs(skill-evals): add a convention for recording live eval runs

[thisisnic]

Summary

• Draft for design feedback before more families get logged.
• The eval suites are never run against a live model in CI, so there was no record of which suites had been run on which model, or how they fared. Adds a short Recording runs convention to the harness README: a per-skill Run history table and four rules.
• Seeds the first table in issue-reassess-stats with the pre-fix flake observed on main that fix(issue-reassess-stats): pin nature counting in step-2 classify eval spec #1615 addresses.

Design questions I'd like feedback on

• Per-skill tables only, no cross-skill summary page. An earlier draft had a hand-maintained docs/eval-run-log.md listing all 81 suites; it was either a duplicate of the per-skill tables or a lossy digest, so it was dropped.
• Refs are SHAs on main only; branch runs aren't logged. Simpler than tracking branch SHAs that rebases rewrite.
• Model names are whatever the CLI calls them, no cross-provider normalisation.
• Rows are append-only. No pruning rules yet; happy to add them if the tables get long.

Type of change

• Documentation (docs/, README.md, CONTRIBUTING.md)
• Skill change
• Tool / bridge contract
• Python package
• Groovy reference impl
• Cross-cutting
• Project template
• CI / dev loop
• Other:

Test plan

• prek run passes on the changed files
• prek run --all-files
• N/A for evals: no skill or fixture text changes

RFC-AI-0004 compliance

No principles touched — documentation only.

Linked issues

Refs #1615 (the fix the seeded row points at).

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[thisisnic]

@potiuk - I was going to run some more of these but was after a bit of feedback on the design of this before I get into it more. I've kept it intentionally minimal for the moment, and was a bit hesitant to add a summary page as it'd need to keep in sync with the individuals suites, but alternatively, I could ditch the individual ones in favour of an overall summary page - what do you think?