refactor: rename assert: to assertions: in EVAL.yaml schema

[christso]

Objective

Rename the assert: key to assertions: across the EVAL.yaml schema, parser, validator, types, docs, and all examples.

Motivation

• assertions: reads naturally as a list key — assert: sounds imperative and collides with the assert statement keyword in JavaScript and Python, which is confusing in code-judge and test contexts.
• Renaming pre-v1 is cheap; post-v1 with external eval files in the wild it becomes a breaking migration.

Scope

All occurrences of the YAML key assert: used as an evaluator list in eval files must become assertions:. This includes:

• Schema definition and validation (eval-file.schema.ts or equivalent)
• YAML parser / loader
• TypeScript types (EvalTest, EvalConfig, etc.)
• Builtin evaluator registry and evaluator parser
• All *.yaml / *.yml eval files under examples/
• All test fixtures and unit tests that reference the key
• Documentation, README, and skill docs that show EVAL.yaml snippets
• The agentv validate command output if it references the key by name

Design Latitude

• Simple find-and-replace across the repo is acceptable — no API redesign needed.
• Add a deprecation warning (parse assert: as assertions: with a console warning) if backwards compatibility during transition is desired; otherwise a hard rename is fine.
• Update agentv validate error messages to use assertions: in examples.

Acceptance Signals

• assert: no longer appears as a schema key in any eval file, type, or validator
• assertions: is accepted by agentv validate without errors
• All existing tests pass with the renamed key
• Examples under examples/ all use assertions:
• agentv validate error output references assertions: not assert:

Non-Goals

• Renaming assert in TypeScript code (variable names, test assertions, etc.) — only the YAML key
• Changes to evaluator internals or evaluation logic

Related

• feat: create EVAL.yaml to evals.json transpiler #598 — EVAL.yaml → evals.json transpiler (update field mapping table: assert → assertions)
• feat: input_files shorthand in EVAL.yaml test cases #602 — input_files: shorthand