HOK-1596: Fix router to aggregate evals from global wavemill install dir

[timogilvie]

Summary

• When running in a non-wavemill repo (e.g. gtm-backend), the router only found records from that repo's per-repo eval file, not the cross-repo aggregated data stored in the wavemill installation directory
• Added resolveGlobalAggregatedEvalsPath() in evals-paths.ts that resolves to <wavemill-install>/.wavemill/evals/aggregated-evals.jsonl using import.meta.url to navigate up from shared/lib/
• The router's loadMergedEvalRecords() now tries both the per-repo aggregated path and the global wavemill install path, deduplicating records by ID across both sources
• Added WAVEMILL_AGGREGATED_EVALS_PATH and WAVEMILL_DIR env overrides as test hooks for the new path resolution

Changes

• shared/lib/evals-paths.ts — Added resolveWavemillInstallDir() and resolveGlobalAggregatedEvalsPath() with env overrides for testability
• shared/lib/model-router.ts — Updated loadMergedEvalRecords() to iterate candidate paths (per-repo + global), merging and deduplicating records from all found sources
• shared/lib/check-routing.ts — Added global aggregated path to the router health check readSource entries
• shared/lib/evals-paths.test.ts — Tests for new path resolution functions
• shared/lib/model-router.test.ts — Tests covering global fallback, deduplication, missing-file resilience, and error handling
• shared/lib/stage-aware-router.test.ts — Tests for stage-aware router with global aggregated evals
• shared/lib/check-routing.test.ts — Tests for health check with global aggregated source

Test plan

• Unit tests cover: global path fallback when per-repo path missing, deduplication of records appearing in both sources, graceful handling when neither path exists, parse errors in one source don't prevent loading from the other
• WAVEMILL_AGGREGATED_EVALS_PATH env override allows tests to inject a fixture file without touching the real wavemill install

Self-review

• Verdict: ready (exit code 0, 1 iteration)
• 4 warnings (no blockers):
  • check-routing.ts:169 — Two sources share the aggregated label; could use aggregated-global to avoid ambiguity (cosmetic)
  • model-router.test.ts:95 — process.chdir() is fragile if tests are parallelized (currently sequential, safe)
  • Plan compliance: eval-aggregator.ts not updated (writer still writes to per-repo path; global reads from wavemill install where eval-aggregate skill writes)
  • Plan compliance: .wavemill/context/router.md docs not updated in this diff