refactor: remove Bun artifacts and convert remaining Bun APIs (#1002)

## Phase 5: Bun Cleanup

Final phase of the Bun → Node.js migration. Removes Bun-specific
artifacts and converts the last remaining Bun API usages.

### Changes

**Removed Bun artifacts:**
- `bun.lock` and `docs/bun.lock` — replaced by `pnpm-lock.yaml` (added
in Phase 4)
- `bunfig.toml` — Bun configuration, no longer needed
- `@types/bun` from `devDependencies` — replaced by `src/global.d.ts`
- `.cursor/rules/bun-cli.mdc` — outdated Bun-specific Cursor AI rule

**Converted remaining Bun APIs:**
- `src/commands/local/run.ts`: `Bun.spawn()` → `spawn()` from
`node:child_process`
- `src/commands/local/server.ts`: `Bun.sleep()` → `sleep()` from
`node:timers/promises`
- Removed corresponding `spawn`/`sleep` polyfills from
`script/node-polyfills.ts`

**Added type declarations (`src/global.d.ts`):**
- `Worker` class with typed event map (previously from `@types/bun`)
- `RegExp.escape()` augmentation (ES2025, Node 23.6+)

**Test fixes:**
- `test/commands/local/run.test.ts`: removed `isBun` skip guard (now
runs on Node)
- `test/lib/db/concurrent.test.ts`: removed `isBun` skip guard, switched
worker spawn to `tsx`
- `test/script/node-polyfills.test.ts`: removed spawn polyfill tests

### Intentionally kept
- `bun:sqlite` try/catch in `src/lib/db/sqlite.ts` — compiled Bun binary
still needs it
- `node-polyfills.ts` — still used by `bundle.ts` for the npm CJS
distribution
- `Bun.build()` in `script/build.ts` — no Node equivalent for binary
compilation
- `setup-bun` in `build-binary` CI job

Author: BYK

.cursor/rules/bun-cli.mdc

@@ -10,7 +10,10 @@
     "./lint-rules/no-stderr-write-in-commands.grit"
   ],
   "files": {
-    "includes": ["!docs", "!test/init-eval/templates"]
+    // custom-ca.ts excluded: Biome's type analysis hits the 200k type limit
+    // on the node:tls module graph — an internal Biome bug that surfaces
+    // non-deterministically as error vs warning. See biome issue tracker.
+    "includes": ["!docs", "!test/init-eval/templates", "!!src/lib/custom-ca.ts"]
   },
   "javascript": {
     "globals": ["Bun"]