fix: harden error handling, fix TOCTOU races, and improve test accuracy

Core library fixes:
- session-manager.js: wrap all statSync calls in try-catch to prevent
  TOCTOU crashes when files are deleted between readdir and stat
- session-manager.js: use birthtime||ctime fallback for Linux compat
- session-manager.js: remove redundant existsSync before readFile
- utils.js: fix findFiles TOCTOU race on statSync inside readdir loop

Hook improvements:
- Add 1MB stdin buffer limits to all PostToolUse hooks to prevent
  unbounded memory growth from large payloads
- suggest-compact.js: use fd-based atomic read+write for counter file
  to reduce race window between concurrent invocations
- session-end.js: log when transcript file is missing, check
  replaceInFile return value for failed timestamp updates
- start-observer.sh: log claude CLI failures instead of silently
  swallowing them, check observations file exists before analysis

Test fixes:
- Fix blocking hook tests to send matching input (dev server command)
  and expect correct exit code 2 instead of 1

scripts/hooks/suggest-compact.js

@@ -13,10 +13,10 @@
  * - Compact after completing a milestone, before starting next
  */
 
+const fs = require('fs');
 const path = require('path');
 const {
   getTempDir,
-  readFile,
   writeFile,
   log
 } = require('../lib/utils');
@@ -32,16 +32,28 @@ async function main() {
   let count = 1;
 
   // Read existing count or start at 1
-  const existing = readFile(counterFile);
-  if (existing) {
-    const parsed = parseInt(existing.trim(), 10);
-    // Guard against NaN from corrupted counter file
-    count = Number.isFinite(parsed) ? parsed + 1 : 1;
+  // Use fd-based read+write to reduce (but not eliminate) race window
+  // between concurrent hook invocations
+  try {
+    const fd = fs.openSync(counterFile, 'a+');
+    try {
+      const buf = Buffer.alloc(64);
+      const bytesRead = fs.readSync(fd, buf, 0, 64, 0);
+      if (bytesRead > 0) {
+        const parsed = parseInt(buf.toString('utf8', 0, bytesRead).trim(), 10);
+        count = Number.isFinite(parsed) ? parsed + 1 : 1;
+      }
+      // Truncate and write new value
+      fs.ftruncateSync(fd, 0);
+      fs.writeSync(fd, String(count), 0);
+    } finally {
+      fs.closeSync(fd);
+    }
+  } catch {
+    // Fallback: just use writeFile if fd operations fail
+    writeFile(counterFile, String(count));
   }
 
-  // Save updated count
-  writeFile(counterFile, String(count));
-
   // Suggest compact after threshold tool calls
   if (count === threshold) {
     log(`[StrategicCompact] ${threshold} tool calls reached - consider /compact if transitioning phases`);