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>

1 files changed, 28 insertions, 4 deletions

diff --git a/README.md b/README.md
index 30b5db8..6ffbdb7 100644
--- a/README.md
+++ b/README.md
@@ -63,6 +63,10 @@ health:
 logs:
   - /path/to/logs/current.log      # up to 4 paths; symlinks resolved
 
+# OR: watch a directory for new log files (mutually exclusive with logs:)
+log_dir: /path/to/logs             # directory to watch
+log_pattern: "L???????.log"        # fnmatch glob; default "*" if omitted
+
 # Optional filter — any executable reading stdin, writing stdout
 log_filter: /usr/lib/umbrella/filters/source.py
 
@@ -86,19 +90,39 @@ stdout after each write works. Python, shell, compiled binary — anything.
 
 ## Log file setup
 
-srcds and most dedicated servers create new log files per session.
-Point `logs:` at a stable symlink and update it on each server start:
+Two modes are available. They are mutually exclusive per unit.
+
+### Fixed-path mode (`logs:`)
+
+Point `logs:` at a stable symlink; umbrella resolves it via `realpath()`
+at startup and watches the real inode.
 
 ```bash
 # In your startup script / ExecStartPre:
 ln -sf /srv/game/logs/L0222000.log /srv/game/logs/current.log
-# Then SIGHUP umbrella so it re-resolves the symlink:
+# SIGHUP umbrella so it re-resolves the symlink:
 kill -HUP $(cat /run/umbrella/umbrella.pid)
 ```
 
-With `sv_log_onefile 1` (Source engine), the log file is fixed for the
+With `sv_log_onefile 1` (Source engine), the log file is stable for the
 server's lifetime, so the symlink only needs updating on restarts.
 
+### Directory-watch mode (`log_dir` + `log_pattern`)
+
+Valve games (TF2, GMod) create a new timestamped log file on every map
+change (`L0222000.log`, `L0222001.log`, ...). Use `log_dir` so umbrella
+auto-switches to the newest matching file without a SIGHUP:
+
+```yaml
+log_dir: /home/gmod/nnn/gmodds/garrysmod/logs
+log_pattern: "L???????.log"
+```
+
+- `log_pattern` uses `fnmatch(3)` shell-style globs (e.g. `L???????.log`)
+- Omitting `log_pattern` defaults to `"*"` (match all files)
+- `log_dir` and `logs:` cannot be combined; if both appear, `logs:` is
+  ignored with a warning in the daemon log
+
 ## CLI usage
 
 ```bash