docs: add build_logs.sqlite section to persistence.md, trim build_logs.rs module doc

docs/persistence.md

@@ -127,6 +127,28 @@ sentinel files (`hyperhive-rate-limited`, `hyperhive-needs-login`) if the
 JSON is absent, so existing containers keep working through the transition
 window before their next rebuild.
 
+### `/var/lib/hyperhive/build_logs.sqlite` (host)
+
+Full stdout + stderr capture for every `nixos-container` / `nix
+build` invocation the lifecycle layer fires. One row per invocation;
+the row accumulates lines as the child runs.
+
+Replaces the legacy 32-line stderr ring buffer that `lifecycle::run`
+kept. The ring tail routinely truncated real eval errors ("tried
+alternatives" blocks alone are often 30+ lines), so failures bailed
+with an arbitrary tail whose full stream only lived in the host
+journal. With this table the dashboard can surface the entire log.
+
+Two indices:
+- `(agent, started_at)` — backs the per-agent latest-N lookup used
+  by the agent card chip.
+- `(status, finished_at)` — backs the retention sweep that runs
+  as part of the existing hourly vacuum.
+
+Writes are best-effort: `append_stdout` / `append_stderr` / `finish`
+log a warning on sqlite error and let the build continue. A failed
+log row never blocks a rebuild.
+
 ### `/state/hyperhive-model` (per agent)
 
 Single-line text file holding the claude model name currently