feat(gym): add multi-file-refactor and flaky-test-triage scenarios

[rysweet]

Extends the gym benchmark suite with two more diverse scenarios beyond the bug-localization and long-context entries that landed in 1825941.

Scenarios added

• multi-file-refactor-rename-symbol (Refactoring class, simard-engineer identity, local-harness, SingleProcess) — asks the agent to plan a behaviour-preserving rename of the public type BenchmarkScenario to GymScenario across the crate. The agent must enumerate call sites, propose an ordered edit sequence, and list verification gates (cargo check, cargo test --lib -- gym, cargo clippy --all-targets) without actually performing the rename.
• flaky-test-triage-from-history (Debugging class, simard-engineer identity, local-harness, SingleProcess) — supplies a 6-run intermittent CI history for a single Rust integration test and asks the agent to classify the flake category (timing/race, ordering/iteration, external-dependency, resource-leak, or rng-seed), state a root-cause hypothesis, and propose a stabilisation plus verification strategy.

Both scenarios follow the same pattern as the bug-localize-from-cargo-test-output scenario added in 1825941 (verbatim evidence block in objective, structured short-form output contract).

Mechanics

• Bumps BENCHMARK_SCENARIOS length from 149 → 151 in src/gym/scenarios.rs.
• Updates the count assertion in src/gym/tests_scenarios.rs to 151.
• cargo test --lib -- gym passes locally (418 passed, 0 failed).

Refs #1210

[rysweet]

Closing as outdated. This PR was opened before #1221 / #1225 landed; rebasing it onto current main would delete the scenarios already added by those merged PRs (memory-recall-consolidated-episode, multi-file-rename-public-api, plan-decomposition-vague-goal, failure-recovery-dirty-worktree). The two scenarios this PR uniquely adds (multi-file-refactor-rename-symbol, flaky-test-triage-from-history) overlap conceptually with what's already in tree; if the daemon decides they're still wanted, she'll re-spawn them on top of current main.