fix(hermes): attribute mixed-session usage by model

[pitimon]

Summary

• read Hermes session_model_usage when available and aggregate usage by (session, model) across billing/task dimensions
• track incremental snapshots independently per session and model
• use each model's authoritative last_seen timestamp for bucket placement
• preserve the legacy aggregate-session parser for older Hermes databases
• baseline existing legacy cursors once to avoid replaying lifetime totals during upgrade
• add mixed-model Sol/Terra regression coverage with later Terra-only growth
• bump release metadata to 0.39.48

Root cause

Hermes records mixed-model sessions in session_model_usage, while TokenTracker only read aggregate counters plus the single sessions.model value. Tokens produced after an in-session model switch were therefore counted but attributed to the original/default model.

Verification

• regression failed before implementation (eventsAggregated 1 instead of 2)
• mixed-model initial totals split correctly
• later Terra growth emits only under Terra
• older Hermes schema fallback tests pass
• named profile isolation tests pass
• live legacy-cursor dry run establishes per-model snapshots with zero replay/queue writes
• all Hermes-focused tests pass
• npm run ci:local passes
• version lockstep, OpenWiki, pack/publish dry run, diff checks pass

Migration note

Existing aggregate cursors are baselined once to avoid double-counting. Previously misattributed historical tokens remain in their old model bucket; new per-model growth is attributed correctly after adoption.

Closes #169

[pitimon]

Release and live verification complete:

• published npm @ipv9/tokentracker-cli@0.39.48 (sha1 a1438e9e2d545382d86947263eb086624f2188db)
• GitHub release v0.39.48 with macOS/Windows assets
• local dashboard and sync LaunchAgents pinned to 0.39.48; plist lint clean; processes healthy; sync exit 0; HTTP 200
• live source session_model_usage -> per-model cursor -> queue -> API -> Day UI verified
• Day UI now shows HERMES 2 models: gpt-5.6-sol 99.4% and gpt-5.6-terra 0.6%
• live API showed Terra 1,125,246 tokens and $0.767189 for 2026-08-06 at verification time

Follow-up publish snapshot PR #171 merged.