General: Remove Nebula subproject

Delete the nebula/ directory entirely and strip all remaining
references from CI workflows, labelers, renovate config, pre-commit
config, docs, and the top-level README. Also drops Nebula from the
PR title regex in validate-pr-title.yml and logos/AGENTS.md.

Minor follow-ups in iris/: rename the default transcription temp_dir
from /tmp/nebula-transcription to /tmp/iris-transcription, and drop
a stale "from Nebula production code" attribution comment in
slide_turn_detector.py.

Author: bassner

.github/issue-labeler.yml

@@ -5,7 +5,4 @@ atlas:
   - atlas
 
 iris:
-  - iris
-
-nebula:
-  - nebula
+  - iris

.github/pullrequest-labeler.yml

@@ -14,10 +14,6 @@ iris:
   - changed-files:
       - any-glob-to-any-file: iris/**/*
 
-nebula:
-  - changed-files:
-      - any-glob-to-any-file: nebula/**/*
-
 memiris:
   - changed-files:
       - any-glob-to-any-file: memiris/**/*