fix: guard session_ids with optional chaining to prevent crash

boulderState?.session_ids.includes() only guards boulderState, not
session_ids. If boulder.json is corrupted or missing the field,
session_ids is undefined and .includes() crashes silently, losing
subagent results.

Changes:
- readBoulderState: validate parsed JSON is object, default session_ids to []
- atlas hook line 427: boulderState?.session_ids?.includes
- atlas hook line 655: boulderState?.session_ids?.includes
- prometheus-md-only line 93: boulderState?.session_ids?.includes
- appendSessionId: guard with ?. and initialize to [] if missing

Fixes #1672

src/hooks/atlas/tool-execute-after.ts

@@ -65,7 +65,7 @@ export function createToolExecuteAfterHandler(input: {
       if (boulderState) {
         const progress = getPlanProgress(boulderState.active_plan)
 
-        if (toolInput.sessionID && !boulderState.session_ids.includes(toolInput.sessionID)) {
+        if (toolInput.sessionID && !boulderState.session_ids?.includes(toolInput.sessionID)) {
           appendSessionId(ctx.directory, toolInput.sessionID)
           log(`[${HOOK_NAME}] Appended session to boulder`, {
             sessionID: toolInput.sessionID,