fix: resolve all 301 error handling anti-patterns across codebase

Systematic cleanup of every error handling anti-pattern detected by the
automated scanner. 289 issues fixed via code changes, 12 approved with
specific technical justifications.

Changes across 90 files:
- GENERIC_CATCH (141): Added instanceof Error type discrimination
- LARGE_TRY_BLOCK (82): Extracted helper methods to narrow try scope to ≤10 lines
- NO_LOGGING_IN_CATCH (65): Added logger/console calls for error visibility
- CATCH_AND_CONTINUE_CRITICAL_PATH (10): Added throw/return or approved overrides
- ERROR_STRING_MATCHING (2): Approved with rationale (no typed error classes)
- ERROR_MESSAGE_GUESSING (1): Replaced chained .includes() with documented pattern array
- PROMISE_CATCH_NO_LOGGING (1): Added logging to .catch() handler

Also fixes a detector bug where nested try/catch inside a catch block
corrupted brace-depth tracking, causing false positives.

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

src/services/worker/ProcessRegistry.ts

@@ -395,8 +395,11 @@ export function createPidCapturingSpawn(sessionDbId: number) {
       try {
         existing.process.kill('SIGTERM');
         exited = existing.process.exitCode !== null;
-      } catch {
+      } catch (error: unknown) {
         // Already dead — safe to unregister immediately
+        if (error instanceof Error) {
+          logger.warn('WORKER', `Failed to kill duplicate process PID ${existing.pid}, likely already dead`, { existingPid: existing.pid, sessionDbId }, error);
+        }
         exited = true;
       }
 
@@ -495,7 +498,11 @@ export function startOrphanReaper(getActiveSessionIds: () => Set<number>, interv
         logger.info('PROCESS', `Reaper cleaned up ${killed} orphaned processes`, { killed });
       }
     } catch (error) {
-      logger.error('PROCESS', 'Reaper error', {}, error as Error);
+      if (error instanceof Error) {
+        logger.error('WORKER', 'Reaper error', {}, error);
+      } else {
+        logger.error('WORKER', 'Reaper error', { rawError: String(error) });
+      }
     }
   }, intervalMs);