Biome: tighten CI gate to fail on warnings (burn down the ~521 warning backlog) #625

Context

#624 made Biome a CI gate at level 1: pnpm run code:check (biome check) runs in tests.yml and fails on errors only. Warnings are allowed through, so the tree currently carries a large warning backlog that is visible but non-blocking.

As of #624 landing, biome check . reports roughly:

• ~521 warnings, dominated by:
  • lint/style/noNonNullAssertion (~298)
  • plugin — the no-type-erasing-assertions rule, as any/never/unknown in source (~82)
  • lint/suspicious/noExplicitAny (~72)
  • lint/correctness/noUnusedImports (~30), noUnusedVariables (~22), unused suppressions (~14), and a long tail.
• 0 errors (kept clean by the level-1 gate).

Goal

Tighten the gate to fail on warnings too — i.e. code:check becomes biome check --error-on-warnings (or equivalent), so no new warnings can land. This requires burning the backlog to zero first.

Work

1. Triage the ~521 warnings by rule. For each: auto-fix where safe (biome check --write handles many), otherwise fix by hand or add a justified // biome-ignore with a reason.
   • The no-type-erasing-assertions (~82) and noExplicitAny (~72) ones are the most valuable — they mark real type-safety holes. Some (e.g. the structural-client force-casts) are legitimately unavoidable and should get an ignore-with-reason.
2. Flip no-type-erasing-assertions from warn to error in the plugin (optional but recommended once the 82 are cleared).
3. Change the CI step / code:check to fail on warnings.
4. Confirm biome check exits 0 cleanly and the gate holds.

Notes

• Do before the next release (per discussion).
• Generated files are already excluded from Biome in chore(lint): warn on type-erasing assertions (as any/never/unknown) via Biome plugin #624 (biome.json files.includes), so the backlog is authored code only.
• Level 2 (flip just the assertion plugin to error) is a valid smaller intermediate step if the full sweep is too big at once.

https://claude.ai/code/session_01MxTTPaPP16m6br7Hoab94w