refactor(adf): split bundler orchestration from pure merge logic

[SmartBrandStrategies]

Linked feedback: ADX-007

Problem

packages/adf/src/bundler.ts currently combines manifest parsing, trigger resolution,
AST merge/transformation, token estimation, and orchestration/file loading concerns.
This raises cognitive load and couples pure transformation behavior to orchestration paths.

Evidence:

• Manifest parsing in bundler: packages/adf/src/bundler.ts:29
• Trigger resolution in bundler: packages/adf/src/bundler.ts:160
• Merge and token estimation in bundler: packages/adf/src/bundler.ts:331

Scope

• Extract manifest parsing/trigger resolution to packages/adf/src/manifest.ts.
• Extract pure merge/token-estimation functions to packages/adf/src/merger.ts.
• Keep bundler.ts as orchestration shell only:
  • load manifest
  • resolve module paths
  • read content
  • parse docs
  • call pure merger/evaluator

Acceptance Criteria

• Merge utilities are pure and independently unit-tested without filesystem mocks.
• bundler.ts no longer defines merge/token estimation internals.
• CLI behavior for adf bundle is unchanged for existing fixtures/tests.
• Existing bundle tests pass; add focused tests for extracted merger.ts.

Test Plan

• pnpm run test --filter @stackbilt/adf
• Add unit tests for:
  • duplicate-section merge behavior across content types
  • weight promotion rules
  • token estimate parity against current behavior

[SmartBrandStrategies]

Blocks #9 — the unified evidence pipeline depends on the clean merge/manifest boundaries this refactor introduces.