Implement the estimation foundations for pre-flight command cost previews.

Concrete files to investigate and update:

• pdd/server/token_counter.py
• pdd/prompts/server/token_counter_python.prompt
• pdd/track_cost.py
• pdd/prompts/track_cost_python.prompt
• .pdd/llm_model.csv
• existing token/cost tests under tests/

Scope:

• Extend model pricing loading so .pdd/llm_model.csv reads both input and output pricing.
• Add an output-inclusive cost estimate API such as estimate_completion_cost(input_tokens, predicted_output_tokens, model, pricing_csv).
• Extend CostEstimate to expose input, output, and total cost while preserving existing callers.
• Preserve unknown-pricing behavior: token counts/context can still be reported, but cost must be explicitly unknown.
• Extend cost logging in pdd/track_cost.py so actual input_tokens and output_tokens can be recorded while preserving legacy CSV/header handling.

PDD Command Hint: change+sync

Acceptance criteria:

• pdd/server/token_counter.py can compute deterministic input+output pricing from a fixture pricing CSV.
• Unknown models return token metrics without silently inventing a price.
• pdd/track_cost.py remains backward-compatible with legacy cost CSVs and supports new token-count columns.

Split Contract

Command sequence: change → sync
Allowed write set:

• pdd/server/token_counter.py
• pdd/prompts/server/token_counter_python.prompt
• pdd/track_cost.py
• pdd/prompts/track_cost_python.prompt
• tests/test_token_counter.py
• tests/test_track_cost.py
  Acceptance criteria:
• Token-counter tests cover input and output rates from a fixture .pdd/llm_model.csv.
• Unknown model pricing reports unknown cost rather than a guessed dollar value.
• Cost logging accepts and preserves input_tokens and output_tokens columns without breaking legacy CSV handling.
  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