log_tail: add log_dir + log_pattern directory-watch mode

Valve games (TF2, GMod) rotate into a new timestamped log file on every
map change. The existing fixed-path inotify watch goes stale after the
first rotation. This adds a directory-watch mode that auto-switches to
the newest matching file whenever one appears.

New YAML fields (mutually exclusive with logs:):
  log_dir:     directory to watch for new log files
  log_pattern: fnmatch(3) glob for filenames; default "*"

Changes:
- umbrella.h: add log_dir[MAX_PATH] and log_dir_pattern[MAX_PATH] to Unit
- log_tail.c: extend LogWatch with dir_wd/dir_path/pattern fields;
  add log_tail_drain, log_tail_scan_dir, log_tail_switch_file,
  log_tail_open_fixed_watch, log_tail_open_dir_watch,
  log_tail_reopen_fixed, log_tail_handle_rotation_dir;
  refactor log_tail_init, log_tail_handle, log_tail_cleanup
- unit.c: parse log_dir and log_pattern YAML keys; warn and drop logs:
  if both are set on the same unit
- AGENTS.md, README.md: document both log-tail modes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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