feat(shared): add dynamic-truncator utility for context-aware output truncation

Extract and generalize dynamic output truncation logic from grep-output-truncator.
Provides context window-aware truncation that adapts based on remaining tokens.

Features:
- truncateToTokenLimit(): Sync truncation with configurable header preservation
- getContextWindowUsage(): Get current context window usage from session
- dynamicTruncate(): Async truncation that queries context window state
- createDynamicTruncator(): Factory for creating truncator instance

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/shared/dynamic-truncator.ts

@@ -0,0 +1,164 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+
+const ANTHROPIC_ACTUAL_LIMIT = 200_000
+const CHARS_PER_TOKEN_ESTIMATE = 4
+const DEFAULT_TARGET_MAX_TOKENS = 50_000
+
+interface AssistantMessageInfo {
+  role: "assistant"
+  tokens: {
+    input: number
+    output: number
+    reasoning: number
+    cache: { read: number; write: number }
+  }
+}
+
+interface MessageWrapper {
+  info: { role: string } & Partial<AssistantMessageInfo>
+}
+
+export interface TruncationResult {
+  result: string
+  truncated: boolean
+  removedCount?: number
+}
+
+export interface TruncationOptions {
+  targetMaxTokens?: number
+  preserveHeaderLines?: number
+  contextWindowLimit?: number
+}
+
+function estimateTokens(text: string): number {
+  return Math.ceil(text.length / CHARS_PER_TOKEN_ESTIMATE)
+}
+
+export function truncateToTokenLimit(
+  output: string,
+  maxTokens: number,
+  preserveHeaderLines = 3
+): TruncationResult {
+  const currentTokens = estimateTokens(output)
+
+  if (currentTokens <= maxTokens) {
+    return { result: output, truncated: false }
+  }
+
+  const lines = output.split("\n")
+
+  if (lines.length <= preserveHeaderLines) {
+    const maxChars = maxTokens * CHARS_PER_TOKEN_ESTIMATE
+    return {
+      result: output.slice(0, maxChars) + "\n\n[Output truncated due to context window limit]",
+      truncated: true,
+    }
+  }
+
+  const headerLines = lines.slice(0, preserveHeaderLines)
+  const contentLines = lines.slice(preserveHeaderLines)
+
+  const headerText = headerLines.join("\n")
+  const headerTokens = estimateTokens(headerText)
+  const truncationMessageTokens = 50
+  const availableTokens = maxTokens - headerTokens - truncationMessageTokens
+
+  if (availableTokens <= 0) {
+    return {
+      result: headerText + "\n\n[Content truncated due to context window limit]",
+      truncated: true,
+      removedCount: contentLines.length,
+    }
+  }
+
+  const resultLines: string[] = []
+  let currentTokenCount = 0
+
+  for (const line of contentLines) {
+    const lineTokens = estimateTokens(line + "\n")
+    if (currentTokenCount + lineTokens > availableTokens) {
+      break
+    }
+    resultLines.push(line)
+    currentTokenCount += lineTokens
+  }
+
+  const truncatedContent = [...headerLines, ...resultLines].join("\n")
+  const removedCount = contentLines.length - resultLines.length
+
+  return {
+    result: truncatedContent + `\n\n[${removedCount} more lines truncated due to context window limit]`,
+    truncated: true,
+    removedCount,
+  }
+}
+
+export async function getContextWindowUsage(
+  ctx: PluginInput,
+  sessionID: string
+): Promise<{ usedTokens: number; remainingTokens: number; usagePercentage: number } | null> {
+  try {
+    const response = await ctx.client.session.messages({
+      path: { id: sessionID },
+    })
+
+    const messages = (response.data ?? response) as MessageWrapper[]
+
+    const assistantMessages = messages
+      .filter((m) => m.info.role === "assistant")
+      .map((m) => m.info as AssistantMessageInfo)
+
+    if (assistantMessages.length === 0) return null
+
+    const lastAssistant = assistantMessages[assistantMessages.length - 1]
+    const lastTokens = lastAssistant.tokens
+    const usedTokens = (lastTokens?.input ?? 0) + (lastTokens?.cache?.read ?? 0)
+    const remainingTokens = ANTHROPIC_ACTUAL_LIMIT - usedTokens
+
+    return {
+      usedTokens,
+      remainingTokens,
+      usagePercentage: usedTokens / ANTHROPIC_ACTUAL_LIMIT,
+    }
+  } catch {
+    return null
+  }
+}
+
+export async function dynamicTruncate(
+  ctx: PluginInput,
+  sessionID: string,
+  output: string,
+  options: TruncationOptions = {}
+): Promise<TruncationResult> {
+  const { targetMaxTokens = DEFAULT_TARGET_MAX_TOKENS, preserveHeaderLines = 3 } = options
+
+  const usage = await getContextWindowUsage(ctx, sessionID)
+
+  if (!usage) {
+    return { result: output, truncated: false }
+  }
+
+  const maxOutputTokens = Math.min(usage.remainingTokens * 0.5, targetMaxTokens)
+
+  if (maxOutputTokens <= 0) {
+    return {
+      result: "[Output suppressed - context window exhausted]",
+      truncated: true,
+    }
+  }
+
+  return truncateToTokenLimit(output, maxOutputTokens, preserveHeaderLines)
+}
+
+export function createDynamicTruncator(ctx: PluginInput) {
+  return {
+    truncate: (sessionID: string, output: string, options?: TruncationOptions) =>
+      dynamicTruncate(ctx, sessionID, output, options),
+
+    getUsage: (sessionID: string) => getContextWindowUsage(ctx, sessionID),
+
+    truncateSync: (output: string, maxTokens: number, preserveHeaderLines?: number) =>
+      truncateToTokenLimit(output, maxTokens, preserveHeaderLines),
+  }
+}

src/shared/index.ts

@@ -9,3 +9,4 @@ export * from "./pattern-matcher"
 export * from "./hook-disabled"
 export * from "./deep-merge"
 export * from "./file-utils"
+export * from "./dynamic-truncator"