feat: inject file observation timeline on PreToolUse Read hook

When Claude reads a file, the PreToolUse hook queries for existing
observations about that file and injects the timeline into context
via additionalContext + permissionDecision: allow. This prevents
duplicate observations and saves tokens through active rediscovery.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/cli/handlers/file-context.ts

@@ -0,0 +1,142 @@
+/**
+ * File Context Handler - PreToolUse
+ *
+ * Injects relevant observation history when Claude reads/edits a file,
+ * so it can avoid duplicating past work.
+ */
+
+import type { EventHandler, NormalizedHookInput, HookResult } from '../types.js';
+import { ensureWorkerRunning, workerHttpRequest } from '../../shared/worker-utils.js';
+import { logger } from '../../utils/logger.js';
+import { HOOK_EXIT_CODES } from '../../shared/hook-constants.js';
+import { isProjectExcluded } from '../../utils/project-filter.js';
+import { SettingsDefaultsManager } from '../../shared/SettingsDefaultsManager.js';
+import { USER_SETTINGS_PATH } from '../../shared/paths.js';
+
+const TYPE_ICONS: Record<string, string> = {
+  decision: '\u2696\uFE0F',
+  bugfix: '\uD83D\uDD34',
+  feature: '\uD83D\uDFE3',
+  refactor: '\uD83D\uDD04',
+  discovery: '\uD83D\uDD35',
+  change: '\u2705',
+};
+
+function compactTime(timeStr: string): string {
+  return timeStr.toLowerCase().replace(' am', 'a').replace(' pm', 'p');
+}
+
+function formatTime(epoch: number): string {
+  const date = new Date(epoch);
+  return date.toLocaleString('en-US', { hour: 'numeric', minute: '2-digit', hour12: true });
+}
+
+function formatDate(epoch: number): string {
+  const date = new Date(epoch);
+  return date.toLocaleString('en-US', { month: 'short', day: 'numeric', year: 'numeric' });
+}
+
+interface ObservationRow {
+  id: number;
+  title: string | null;
+  type: string;
+  created_at_epoch: number;
+}
+
+function formatFileTimeline(observations: ObservationRow[], filePath: string): string {
+  // Group observations by day
+  const byDay = new Map<string, ObservationRow[]>();
+  for (const obs of observations) {
+    const day = formatDate(obs.created_at_epoch);
+    if (!byDay.has(day)) {
+      byDay.set(day, []);
+    }
+    byDay.get(day)!.push(obs);
+  }
+
+  // Sort days chronologically
+  const sortedDays = Array.from(byDay.entries()).sort((a, b) => {
+    const aEpoch = a[1][0].created_at_epoch;
+    const bEpoch = b[1][0].created_at_epoch;
+    return aEpoch - bEpoch;
+  });
+
+  const lines: string[] = [
+    `Existing observations for this file \u2014 review via get_observations([IDs]) to avoid duplicates:`,
+  ];
+
+  for (const [day, dayObservations] of sortedDays) {
+    lines.push(`### ${day}`);
+    for (const obs of dayObservations) {
+      const title = obs.title || 'Untitled';
+      const icon = TYPE_ICONS[obs.type] || '\u2753';
+      const time = compactTime(formatTime(obs.created_at_epoch));
+      lines.push(`${obs.id} ${time} ${icon} ${title}`);
+    }
+  }
+
+  return lines.join('\n');
+}
+
+export const fileContextHandler: EventHandler = {
+  async execute(input: NormalizedHookInput): Promise<HookResult> {
+    // Extract file_path from toolInput
+    const toolInput = input.toolInput as Record<string, unknown> | undefined;
+    const filePath = toolInput?.file_path as string | undefined;
+
+    if (!filePath) {
+      return { continue: true, suppressOutput: true };
+    }
+
+    // Check if project is excluded from tracking
+    const settings = SettingsDefaultsManager.loadFromFile(USER_SETTINGS_PATH);
+    if (input.cwd && isProjectExcluded(input.cwd, settings.CLAUDE_MEM_EXCLUDED_PROJECTS)) {
+      logger.debug('HOOK', 'Project excluded from tracking, skipping file context', { cwd: input.cwd });
+      return { continue: true, suppressOutput: true };
+    }
+
+    // Ensure worker is running
+    const workerReady = await ensureWorkerRunning();
+    if (!workerReady) {
+      return { continue: true, suppressOutput: true };
+    }
+
+    // Query worker for observations related to this file
+    try {
+      const queryParams = new URLSearchParams({ path: filePath });
+      if (input.cwd) {
+        queryParams.set('project', input.cwd);
+      }
+
+      const response = await workerHttpRequest(`/api/observations/by-file?${queryParams.toString()}`, {
+        method: 'GET',
+      });
+
+      if (!response.ok) {
+        logger.warn('HOOK', 'File context query failed, skipping', { status: response.status, filePath });
+        return { continue: true, suppressOutput: true };
+      }
+
+      const data = await response.json() as { observations: ObservationRow[]; count: number };
+
+      if (!data.observations || data.observations.length === 0) {
+        return { continue: true, suppressOutput: true };
+      }
+
+      const timeline = formatFileTimeline(data.observations, filePath);
+
+      return {
+        hookSpecificOutput: {
+          hookEventName: 'PreToolUse',
+          permissionDecision: 'allow',
+          additionalContext: timeline,
+        },
+      };
+    } catch (error) {
+      logger.warn('HOOK', 'File context fetch error, skipping', {
+        error: error instanceof Error ? error.message : String(error),
+      });
+      return { continue: true, suppressOutput: true };
+    }
+  },
+};