fix: prevent zombie processes, redact cron stderr, skip symlinks in skill enumeration

process_registry.py: _reader_loop() has process.wait() after the try-except
block (line 380).  If the reader thread crashes with an unexpected exception
(e.g. MemoryError, KeyboardInterrupt), control exits the except handler but
skips wait() — leaving the child as a zombie process.  Move wait() and the
cleanup into a finally block so the child is always reaped.

cron/scheduler.py: _run_job_script() only redacts secrets in stdout on the
SUCCESS path (line 417-421).  When a cron script fails (non-zero exit), both
stdout and stderr are returned WITHOUT redaction (lines 407-413).  A script
that accidentally prints an API key to stderr during a failure would leak it
into the LLM context.  Move redaction before the success/failure branch so
both paths benefit.

skill_commands.py: _build_skill_message() enumerates supporting files using
rglob("*") but only checks is_file() (line 171) without filtering symlinks.
PR #6693 added symlink protection to scan_skill_commands() but missed this
function.  A malicious skill can create symlinks in references/ pointing to
arbitrary files, exposing their paths (and potentially content via skill_view)
to the LLM.  Add is_symlink() check to match the guard in scan_skill_commands.

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

tools/process_registry.py

@@ -396,15 +396,15 @@ class ProcessRegistry:
                         session.output_buffer = session.output_buffer[-session.max_output_chars:]
         except Exception as e:
             logger.debug("Process stdout reader ended: %s", e)
-
-        # Process exited
-        try:
-            session.process.wait(timeout=5)
-        except Exception as e:
-            logger.debug("Process wait timed out or failed: %s", e)
-        session.exited = True
-        session.exit_code = session.process.returncode
-        self._move_to_finished(session)
+        finally:
+            # Always reap the child to prevent zombie processes.
+            try:
+                session.process.wait(timeout=5)
+            except Exception as e:
+                logger.debug("Process wait timed out or failed: %s", e)
+            session.exited = True
+            session.exit_code = session.process.returncode
+            self._move_to_finished(session)
 
     def _env_poller_loop(
         self, session: ProcessSession, env: Any, log_path: str, pid_path: str, exit_path: str