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>

agent/skill_commands.py

@@ -168,7 +168,7 @@ def _build_skill_message(
             subdir_path = skill_dir / subdir
             if subdir_path.exists():
                 for f in sorted(subdir_path.rglob("*")):
-                    if f.is_file():
+                    if f.is_file() and not f.is_symlink():
                         rel = str(f.relative_to(skill_dir))
                         supporting.append(rel)