fix(hoosage): count full Copilot CLI session cost

[wakemeup0]

Problem

Copilot CLI's per-model modelMetrics in session.shutdown are incomplete:

• they restart after context compaction and after an interrupted run that is resumed,
• they leave out sub-agent calls (explore/research agents on other models).

hoosage summed only those, so CLI cost was heavily undercounted. On real data for August 2026 it showed $23.70 while the sessions' own totals add up to $61.08 (one session: $6.55 counted vs. $30.10 real).

Fix

• The session-level cumulative totalNanoAiu (written by session.usage_checkpoint and session.shutdown) is now authoritative for cost.
• Checkpoints emit cost-only increments (0 tokens / 0 requests; those still come from modelMetrics at shutdown), so running sessions show cost and long sessions are spread over the right days.
• At shutdown, per-model entries keep their reported cost (capped to the still-unassigned session cost); any remainder is added to the current model.
• A drop of the session counter (older CLIs restart it on resume) is treated as a new run: earlier runs stay counted and per-model baselines restart.
• Sessions without a session total keep the previous behaviour.
• Entries are immutable, so the append-only shared project sync stays correct.

Verified against the real session-state data: the per-session sum of totals matches exactly (Windows Aug $61.08, WSL $399.01 overall).

Also removes the stale manual "Unreleased" changelog block left below 0.13.0.