docs: initial pragent design and README

Extensible PR review framework: CLI-in-CI delivery, pi SDK agent loop,
deterministic-plus-LLM tiering, committed repo profile, JSONL/OTel analytics.
Design doc records the decisions, cost model, and rollout phases.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011Ye1KNFMkkUtmzTypHXkoK

.gitignore

@@ -0,0 +1,6 @@
+node_modules/
+dist/
+*.token
+.pragent/cache/
+*.jsonl
+.env