coordinator: blank-slate run variant (observer-blank upstream)

Parallel variant of the live run. Points the coordinator at
ella/observer-blank as upstream and uses claude/observer-blank-improvements
as the scratch branch, so this run cannot collide with the live
q-branch-observer → claude/observer-improvements pipeline driven from
ella/claude-coordinator-harness.

Everything else (workspace_for_detector convention, .coordinator state
paths, gates, reviewers) is identical and derives from runtime state.

Author: ellataira

tasks/coordinator/git_ops.py

@@ -17,8 +17,8 @@
 import subprocess
 from pathlib import Path
 
-SCRATCH_BRANCH = "claude/observer-improvements"
-UPSTREAM_BRANCH = "q-branch-observer"
+SCRATCH_BRANCH = "claude/observer-blank-improvements"
+UPSTREAM_BRANCH = "ella/observer-blank"
 # Only comp/observer/ is committable by the coordinator, with the
 # critical EXCEPTION of comp/observer/scenarios/ — that directory holds
 # ground-truth labels (ground_truth.json) and episode windows