test: Replace HELPER_TOOLS with literal names in case files

[jirispilka]

Problem

CONTRIBUTING.md (line 330) says integration tests must use hardcoded tool-name strings so a renamed tool fails the suite instead of silently following the constant. Every existing file in tests/test_kit/cases/ (storage.cases.ts, actors.cases.ts, tools.cases.ts, apps.cases.ts, tasks.cases.ts, payments.cases.ts, registration.cases.ts) uses HELPER_TOOLS.* instead.

Found as a pre-existing review finding on #1423, whose new schedules.cases.ts follows the rule. Too many files to fold into that PR.

Proposed fix

Replace HELPER_TOOLS.X with the literal tool name across tests/test_kit/cases/*.cases.ts, drop the now-unused import, and reconcile the wording in CLAUDE.md ("exception: integration tests") with CONTRIBUTING so both say the same thing. Or, if the maintainers prefer the constants, change CONTRIBUTING instead; either way the doc and the practice should match.

Context

• Where the rule was applied: fix(evals): Make schedule eval runs safe to run in parallel #1423.
• Parent issue: Schedule evals share mutable account state, so runs can't be parallel #1394. May overlap the unfiled backlog in refactor: Unfiled backlog from the 2026-07-08 codebase sweep #1270.

Drafted by Claude Code during the #1394 shepherd run.