fix: track context window size instead of cumulative API tokens

The plugin tracked totalTokens by accumulating (input + output + reasoning)
deltas across all session messages. Since each message's input already
includes the full conversation context, this re-counted prior turns on every
new message, inflating the total by 5-10x compared to what OpenCode displays.

Fix: use Math.max instead of additive delta so totalTokens reflects the
peak context window size. This matches what OpenCode shows users and makes
the budget check intuitive (context window vs. limit).

Also renames all user-facing labels from tracked tokens to context tokens
for clarity, and adds a regression test.

Fixes #5