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>

cron/scheduler.py

@@ -442,6 +442,14 @@ def _run_job_script(script_path: str) -> tuple[bool, str]:
         stdout = (result.stdout or "").strip()
         stderr = (result.stderr or "").strip()
 
+        # Redact secrets from both stdout and stderr before any return path.
+        try:
+            from agent.redact import redact_sensitive_text
+            stdout = redact_sensitive_text(stdout)
+            stderr = redact_sensitive_text(stderr)
+        except Exception:
+            pass
+
         if result.returncode != 0:
             parts = [f"Script exited with code {result.returncode}"]
             if stderr:
@@ -450,13 +458,6 @@ def _run_job_script(script_path: str) -> tuple[bool, str]:
                 parts.append(f"stdout:\n{stdout}")
             return False, "\n".join(parts)
 
-        # Redact any secrets that may appear in script output before
-        # they are injected into the LLM prompt context.
-        try:
-            from agent.redact import redact_sensitive_text
-            stdout = redact_sensitive_text(stdout)
-        except Exception:
-            pass
         return True, stdout
 
     except subprocess.TimeoutExpired: