chore: full Biome lint cleanup — zero errors, zero warnings

- Remove 203 unused biome-ignore suppression comments (noConsole rule is off)
- Apply all FIXABLE transforms: Math.pow->**, parseInt radix,
  noUselessContinue, noUselessUndefinedInitialization, useIndexOf,
  useRegexLiterals, useShorthandFunctionType, noPrototypeBuiltins
- Add targeted suppressions for 31 intentional patterns
- Format all src/ files via Biome (quote style, import line width)
- Result: 0 errors, 0 warnings across 2649 files

src/cli/bg/engines/index.ts

@@ -1,4 +1,9 @@
-export type { BgEngine, BgStartOptions, BgStartResult, SessionEntry } from '../engine.js'
+export type {
+  BgEngine,
+  BgStartOptions,
+  BgStartResult,
+  SessionEntry,
+} from '../engine.js'
 
 export async function selectEngine(): Promise<import('../engine.js').BgEngine> {
   if (process.platform === 'win32') {