feature(1100): Discover .markdown posts and always log run summary

Extend the anniversary matcher to recursively scan both .md and
.markdown posts under _posts/blog, retaining the existing filename
validation and relative-path dedupe identifier so published-state
bookkeeping keeps working.

Drop the zero-counter short-circuit in log_run_summary so the
run_complete line is always emitted on a successful pass. Debug and
manual runs now clearly show that scanning completed and found
nothing instead of going silent.

Update matching, logging, and run-logging tests to cover the new
extension and always-emit contract, and refresh the logging
architecture doc accordingly.

tests/test_logging_setup.py

@@ -61,10 +61,19 @@ def test_log_run_summary_emits_one_line_with_counters() -> None:
     assert payload["posted_ids"] == ["x"]
 
 
-def test_log_run_summary_silent_when_no_matches() -> None:
+def test_log_run_summary_always_emits_summary_line_with_zero_counters() -> None:
     buf = _capture(logging.getLogger("tenbackward"))
     log_run_summary(0, 0, 0, 0, [])
-    assert buf.getvalue() == ""
+    lines = [line for line in buf.getvalue().splitlines() if line.strip()]
+    assert len(lines) == 1
+    payload = json.loads(lines[0])
+    assert payload["message"] == "run complete"
+    assert payload["event"] == "run_complete"
+    assert payload["scanned"] == 0
+    assert payload["matched"] == 0
+    assert payload["posted"] == 0
+    assert payload["skipped"] == 0
+    assert payload["posted_ids"] == []
 
 
 def test_log_startup_emits_single_info_line() -> None: