feat(compression): implement strategy selector logic

Add selectCompressionStrategy(), getEffectiveMode(), checkComboOverride(), shouldAutoTrigger(), applyCompression(). Priority chain: combo override > auto-trigger > default mode > off. Dispatches to lite compression for Phase 1; standard/aggressive/ultra return unchanged body.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

open-sse/services/compression/strategySelector.ts

@@ -0,0 +1,51 @@
+import type { CompressionConfig, CompressionMode, CompressionResult } from "./types";
+import { applyLiteCompression } from "./lite";
+
+export function checkComboOverride(
+  config: CompressionConfig,
+  comboId: string | null
+): CompressionMode | null {
+  if (!comboId || !config.comboOverrides) return null;
+  return config.comboOverrides[comboId] ?? null;
+}
+
+export function shouldAutoTrigger(config: CompressionConfig, estimatedTokens: number): boolean {
+  return config.autoTriggerTokens > 0 && estimatedTokens >= config.autoTriggerTokens;
+}
+
+export function getEffectiveMode(
+  config: CompressionConfig,
+  comboId: string | null,
+  estimatedTokens: number
+): CompressionMode {
+  if (!config.enabled) return "off";
+
+  const comboMode = checkComboOverride(config, comboId);
+  if (comboMode) return comboMode;
+
+  if (shouldAutoTrigger(config, estimatedTokens)) return "lite";
+
+  return config.defaultMode;
+}
+
+export function selectCompressionStrategy(
+  config: CompressionConfig,
+  comboId: string | null,
+  estimatedTokens: number
+): CompressionMode {
+  return getEffectiveMode(config, comboId, estimatedTokens);
+}
+
+export function applyCompression(
+  body: Record<string, unknown>,
+  mode: CompressionMode,
+  options?: { model?: string }
+): CompressionResult {
+  if (mode === "off") {
+    return { body, compressed: false, stats: null };
+  }
+  if (mode === "lite") {
+    return applyLiteCompression(body, options);
+  }
+  return { body, compressed: false, stats: null };
+}