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/24-availability-map.md

@@ -0,0 +1,24 @@
+# US-024 — Enhanced availability map with per-node health details
+
+Status: done
+Priority: Medium
+Stage: Implemented
+
+## Context
+
+`GET /v1/coverage/<model>` returns band-level coverage (start_layer, end_layer, node_count)
+but gives no visibility into which specific nodes are in each band or whether they are alive.
+Operators debugging a split-model deployment need to see node-level health at a glance.
+
+## Format decision
+
+Both: band metadata + node list grouped under each band. Dead nodes included with `healthy: false`
+so operators can see them.
+
+## Acceptance criteria
+
+- [ ] Each band in the response includes `nodes: [{node_id, endpoint, healthy, queue_depth, last_seen_s}]`
+- [ ] `healthy` is `true` iff last heartbeat < `heartbeat_timeout` seconds ago
+- [ ] Existing band fields (`start_layer`, `end_layer`, `node_count`) preserved
+- [ ] Tests updated to check band fields individually (not exact dict comparison)
+- [ ] `python -m pytest` passes