Estimate Codex API cost via ccusage in API-key mode

[mistercrunch]

Summary

• switch Codex $ estimation from Agor-owned model pricing to a defensive ccusage CLI adapter
• only derive Codex USD cost when the session is running with an explicit OpenAI API key; native-auth / ChatGPT-plan sessions still show cost as unavailable
• keep the estimator fail-open: if ccusage is missing, parsing fails, or prior task costs are incomplete, Agor leaves cost unset instead of showing a misleading value
• add focused tests for ccusage session matching, cumulative-session delta math, and fail-open behavior

Validation

• pnpm --dir packages/executor test -- src/sdk-handlers/codex/ccusage.test.ts src/sdk-handlers/codex/normalizer.test.ts
• git diff --check

Notes

• current ccusage Codex support is exposed through its CLI/native binary, not stable JS internals, so this shells out to ccusage codex session --json --offline
• Agor computes per-task cost by subtracting already-stored prior task costs from ccusage's cumulative per-session cost
• if older Codex tasks in the session have token usage but no stored cost yet, new tasks remain N/A until the session has a reliable baseline