docs: rewrite README for GPU-native direction, add GPU feasibility report

Rewrite README to lead with GPU execution as the primary purpose.
Add comprehensive GPU feasibility report synthesizing 101 findings
from 10 parallel research agents. Update .gitignore for worktrees
and spec state files.

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

Author: claude

.gitignore

@@ -10,9 +10,19 @@ build/
 .coverage
 coverage.xml
 htmlcov/
+
+# Compiled binaries from benchmarks
 benchmarks/sum_n
 benchmarks/sum_n_runtime
 benchmarks/sum_n_runtime_O3
+
+# Knowledge base (generated, large)
 kb/data/
+
+# Ralph spec state
 specs/.current-spec
 **/.progress.md
+**/.ralph-state.json
+
+# Claude worktrees
+.claude/worktrees/