Extend estimate mode to multi-step commands where later LLM calls depend on earlier generated outputs.

Concrete files to investigate and update:

• pdd/sync_orchestration.py
• pdd/prompts/sync_orchestration_python.prompt
• pdd/agentic_sync_runner.py
• pdd/prompts/agentic_sync_runner_python.prompt
• pdd/fix_code_loop.py
• pdd/prompts/fix_code_loop_python.prompt
• pdd/fix_error_loop.py
• pdd/prompts/fix_error_loop_python.prompt
• related sync/agentic tests under tests/

Scope:

• Make pdd --estimate --output-cost run.csv sync <module> collect and print a per-step estimate breakdown.
• Label downstream multi-step estimates as approximate because later prompts depend on generated output.
• Estimate the first known step exactly from the actual built messages and extrapolate later steps using documented command ratios or one-round assumptions.
• Preserve the no-provider-call and no-file-write guarantees in estimate mode.

Interface Contract

This sub-issue depends on sub-issue 2. It must consume the estimate mode state exposed by pdd/core/cli.py and pdd/llm_invoke.py, including the per-run estimate collector and sentinel/short-circuit behavior. It must not introduce a second independent estimate-mode mechanism.

PDD Command Hint: change+sync

Acceptance criteria:

• pdd --estimate --output-cost run.csv sync <module> prints a per-step breakdown and an approximate total.
• Multi-step output clearly labels heuristic downstream estimates and their assumption.
• Tests assert sync estimate mode avoids provider calls, generated files, and cost-log writes.

Split Contract

Command sequence: change → sync
Allowed write set:

• pdd/sync_orchestration.py
• pdd/prompts/sync_orchestration_python.prompt
• pdd/agentic_sync_runner.py
• pdd/prompts/agentic_sync_runner_python.prompt
• pdd/fix_code_loop.py
• pdd/prompts/fix_code_loop_python.prompt
• pdd/fix_error_loop.py
• pdd/prompts/fix_error_loop_python.prompt
• tests/test_sync_orchestration.py
• tests/test_agentic_sync_runner.py
  Acceptance criteria:
• Sync estimate mode prints exact first-step estimates and labelled approximate downstream estimates.
• Estimate mode for sync avoids all provider calls and file writes.
• The output includes a total approximate cost and context usage summary.
  Independently mergeable: True
  Scope rule: Do not expand beyond this contract or implement sibling sub-issue work. If the contract is insufficient, report the gap instead.

PDD Command Hint: This is a new feature. Use change → sync (modify prompts, then generate and validate code).

Parent: #1347
Parent issue: #1347