fix: respect ANTHROPIC_1M_CONTEXT and VERTEX_ANTHROPIC_1M_CONTEXT env vars (#450)

- Update preemptive-compaction hook to use 1M limit when env vars set
- Update dynamic-truncator to use 1M limit for output truncation
- Update context-window-monitor to use 1M limit for usage tracking

Previously hardcoded 200k limits caused compaction at 140k tokens even
with 1M context enabled. Now respects env vars consistently with base
opencode implementation.

Fixes compaction triggering too early with Claude Sonnet 4.5 1M context.

Related to anomalyco/opencode#6660

src/shared/dynamic-truncator.ts

@@ -1,6 +1,10 @@
 import type { PluginInput } from "@opencode-ai/plugin";
 
-const ANTHROPIC_ACTUAL_LIMIT = 200_000;
+const ANTHROPIC_ACTUAL_LIMIT =
+  process.env.ANTHROPIC_1M_CONTEXT === "true" ||
+  process.env.VERTEX_ANTHROPIC_1M_CONTEXT === "true"
+    ? 1_000_000
+    : 200_000;
 const CHARS_PER_TOKEN_ESTIMATE = 4;
 const DEFAULT_TARGET_MAX_TOKENS = 50_000;