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.

docs/architecture/logging.md

@@ -56,7 +56,7 @@ Any matching key is replaced with the string `[REDACTED]`.
 | `configure_json_logging(level=INFO)`  | First line of `main.main()`. Idempotent — clears existing handlers.   | n/a            | n/a                |
 | `log_startup(**fields)`               | After config load + `data_dir.mkdir`, before the pipeline pass.       | `INFO`         | `startup`          |
 | `log_error(event, *, exc=None, **fields)` | When a pipeline attempt raises, or when the retry budget is exhausted. | `ERROR`     | the supplied event name; `exc_type` added when `exc` is provided. |
-| `log_run_summary(scanned, matched, posted, skipped, posted_ids)` | Once per successful pass. **Silently skipped** when all counters are zero and no ids were posted. | `INFO`         | `run_complete`     |
+| `log_run_summary(scanned, matched, posted, skipped, posted_ids)` | Once per successful pass. Always emitted (including zero-counter passes) so debug and manual runs clearly show the scan completed. | `INFO`         | `run_complete`     |
 
 ## Startup payload
 
@@ -87,8 +87,9 @@ Each is added to the `startup` event payload under its own key.
 }
 ```
 
-A pass with no candidates emits **no line at all**, matching the
-"silent on no matches" contract.
+A pass with no candidates still emits a single `run_complete`
+summary line with all counters set to zero, so operators can confirm
+the scan ran and found nothing to publish.
 
 # Examples