chore(dev-tooling): adopt canonical Bun+TS baseline

[JRedeker]

Per ~/toolbox/docs/dev-tooling-baseline.md (D1-D7) — first repo migrated as part of optimizeBunTsDevToolingV4 ADV change (smallest-first phasing).

Changes

• PM declaration: packageManager: bun@1.3.14, engines.node: ">=24.0.0"
• Bun types: migrated bun-types → @types/bun + tsconfig types: ["bun"] (per Bun docs 2026 + DefinitelyTyped PR #67827 shim transition)
• ESLint flat config: @eslint/js + typescript-eslint recommended + eslint-config-prettier (untyped baseline; typed linting opt-in per-repo)
• Prettier: defaults (.prettierrc: {}); .prettierignore excludes .adv/, .opencode/, bun.lock, docs/
• Canonical scripts: typecheck, lint, lint:fix, format, format:check, check (aggregate); ci now calls bun run check
• Pre-existing lint fixes (6 errors surfaced by new ESLint):
  • Removed unused imports (ExecuteMorphEditArgs, FailureKind, statSync)
  • Removed unused variable (originalCode in test)
  • Removed redundant true && in NaN-threshold test
  • Simplified regex escapes in scrubSecrets
• Baseline reformatting: prettier defaults applied to existing source files (README.md, instructions/morph-tools.md, src/{constants,execute,imports}.ts, index.{ts,test.ts})

Verification

$ bun run check
$ tsc --noEmit
$ eslint .
$ prettier --check .
Checking formatting...
All matched files use Prettier code style!
$ bun test
bun test v1.3.14 (0d9b296a)

 137 pass
 0 fail
 295 expect() calls
Ran 137 tests across 1 file. [140.00ms]

Avoidances honored

• Test runner stays bun test (not migrated to Vitest)
• No type-checked ESLint rules (recommendedTypeChecked opt-in for future)
• No Biome migration
• No build-tool changes

Refs

• Tracking ADV change: optimizeBunTsDevToolingV4 (proposal/discovery/design/prep approved)
• Baseline doc: ~/toolbox/docs/dev-tooling-baseline.md
• Sourced: Incompatibility for type-checking with bun vitest-dev/vitest#5441 (Bun+Vitest unsupported), typescript-eslint.io/getting-started, Bun docs/typescript, DefinitelyTyped PR #67827