[Bug]: Usage page never scans Codex archived_sessions/, so archived sessions' usage silently disappears

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Use Codex interactively, then archive some sessions (rollouts move from ~/.codex/sessions/ to ~/.codex/archived_sessions/).
2. Open the Usage page for a window covering that work.
3. Compare the reported Codex tokens/cost against the transcripts actually on disk.

Observed behavior

UsageService.resolveTranscriptDirs resolves only <codexHome>/sessions for the Codex provider, so rollouts in archived_sessions/ are never scanned — even though the Codex shadow-home layout elsewhere in the codebase treats archived_sessions as a known shared directory alongside sessions.

On my machine the effect is dramatic:

• ~/.codex/archived_sessions/ holds 2.7 GB, with 148 files modified inside the 30-day window, containing ~6,700 gpt-5.6-sol turn-context records — this is where nearly all of my real interactive Codex work lives.
• The scanned ~/.codex/sessions/ retains only ~300 gpt-5.6-sol records; the rest of it is dominated by codex-auto-review traffic (~31,000 model records).
• The Usage page therefore reports 177M priced Codex tokens ($131.09) for the window; the true interactive figure is several times that.

Compounding it, codex-auto-review has no LiteLLM rate, so the 885M tokens that are scanned price at $0.00 (the display side of that is tracked in #5799). Net result: Codex usage is doubly understated — most real usage is invisible, and most visible usage is unpriced.

Expected behavior

Usage should cover archived sessions: scan archived_sessions/ alongside sessions/. Transcripts are append-only and the existing mtime filter + (size, mtime) scan cache already bound the cost of the extra walk. At minimum, the page should disclose that archived sessions are excluded from the totals.

Start at UsageService.resolveTranscriptDirs in apps/server and inspect the Codex shadow-home layout elsewhere in the codebase, especially how sessions and archived_sessions are treated. Reproduce the Usage page comparison for a window containing archived work. Done means archived_sessions is scanned alongside sessions and archived transcripts contribute to the reported totals without bypassing the existing mtime and scan-cache behavior.