docs: consolidate all docs under docs/ — single source of truth

Move issues (01–29) and PRD from .scratch/distributed-inference-network/
into docs/issues/ and docs/. Update ralph_progress.py DEFAULT_PRD path
and rewrite docs/agents/issue-tracker.md to reflect the new layout.

The distributed_inference_network.egg-info/docs/ mirror is a build
artifact already covered by *.egg-info/ in .gitignore — not committed.

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

docs/issues/23-heartbeat-stats.md

@@ -0,0 +1,26 @@
+# US-023 — Heartbeat stats payload: request counters + dynamic reassignment response
+
+Status: done
+Priority: Medium
+Stage: Implemented
+
+## Context
+
+Node heartbeats are currently empty POSTs. The tracker has no visibility into per-node load,
+making load balancing and assignment decisions blind. Heartbeats should carry cumulative stats.
+The heartbeat response channel is also the natural place for the tracker to deliver reassignment
+instructions without requiring a node restart.
+
+## Acceptance criteria
+
+- [ ] Heartbeat POST body includes: `total_requests`, `failed_requests`, `queue_depth`, `uptime_seconds`, `status`
+- [ ] `TorchNodeServer` tracks the three counters with a `threading.Lock`
+- [ ] Tracker stores the last heartbeat payload per node
+- [ ] Heartbeat response may include `new_assignment: {model, shard_start, shard_end}`; node logs it
+- [ ] Stats survive tracker outage: buffered locally, flushed on next successful heartbeat
+- [ ] `python -m pytest` passes
+
+## Notes
+
+Hot-reload (loading a new shard without restart) is deferred to a future story. The response
+field is wired so trackers can send the signal; nodes log it but don't act yet.