fix: show n/a for brain doc/chunk counts when stats are unavailable

When skillsArtifact is 'unavailable' or 'unknown' (DB unreachable or
query failure), showing 0/0 is misleading — the counts were never read.
Use n/a instead. Apply the same logic to memory stats when memoryDb is
not available.

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

---
Build: pass | Tests: pass — Tests  2061 passed (2061)

src/telegram-commands.ts

@@ -776,8 +776,12 @@ export async function handleBrainCommand(
       (status.skillsArtifactVersion !== 'unknown'
         ? ` (${status.skillsArtifactVersion})`
         : ''),
-    `Docs: ${status.skillsDocumentRows} · Chunks: ${status.skillsChunkRows}`,
-    `Memory: ${status.memoryRows} entries · ${status.memoryEmbeddingRows} embeddings`,
+    status.skillsArtifact !== 'unavailable' && status.skillsArtifact !== 'unknown'
+      ? `Docs: ${status.skillsDocumentRows} · Chunks: ${status.skillsChunkRows}`
+      : `Docs: n/a · Chunks: n/a`,
+    status.memoryDb === 'available'
+      ? `Memory: ${status.memoryRows} entries · ${status.memoryEmbeddingRows} embeddings`
+      : `Memory: n/a`,
   ];
 
   if (issues.length > 0) {