fix(core): remove camelCase target aliases

[christso]

Follow-up to #935.

Summary

• remove camelCase target aliases from target resolution and validation
• reject camelCase target YAML fields instead of warning while preserving snake_case support
• narrow TargetDefinition so the public input type matches the new runtime contract
• update CLI schema and fallback-target tests to assert rejection semantics

Why

#935 deprecated camelCase aliases but intentionally kept them working. This change completes that migration by enforcing snake_case-only target config.

Verification

• bun test packages/core/test/evaluation/providers/targets.test.ts packages/core/test/evaluation/providers/cli-schema.test.ts packages/core/test/evaluation/validation/targets-validator.test.ts
• live provider smoke via AgentV resolver/provider path with snake_case target definitions:
  • azure -> returned OK
  • claude-cli -> returned OK
  • copilot-cli -> returned OK
• note: local pre-push hook lint was flaky in this worktree and appeared to read the primary checkout's targets.ts; bun run lint passed in the branch worktree before push

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