fix(host_agent): every Docker read is bounded, and the agent reports its own footprint (#5764)

#5764 "The agent reads a container's whole log backlog into memory, on its
main loop, with no overall deadline". Docker03's 1.13.0 agent went silent
for a day at 2.1 GB and 46 h of CPU, then ignored SIGTERM.

- One shared reader, _docker_get: each answer has an overall deadline and a
  byte cap, not only a per-recv timeout. A trickling or endless body is cut
  off (15 s / 32 MiB for JSON, 5 s / 4 MiB for a log read).
- Container logs: since the cursor but only the newest 2,000 lines, all
  within 10 s. A note says when older lines were skipped, and a container
  whose answer is too big skips ahead to now instead of re-reading its
  backlog every interval.
- The agent reports its own RSS and CPU; Steward records agent_rss_bytes
  and agent_cpu_pct, so a runaway agent shows before it goes silent.

Agent 1.15.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plugins/host-agent-design.md

@@ -120,6 +120,10 @@ Python 3.8+ stdlib only. `urllib.request` for the POST, plus `json`, `os`, `shut
 
 Each mount is read in a daemon thread under one 5 s deadline. A hard-mounted NFS share whose server stops blocks `statvfs` in the kernel with no timeout; read inline, that froze the whole agent. A mount that misses the deadline is sent as `{"mount", "stalled": true}`, and isn't probed again while its last probe is still blocked. Steward counts them as `mounts_stalled`.
 
+### Every Docker read is bounded (1.15.0, #5764)
+
+One Docker answer must arrive within 15 s and fit in 32 MiB, and one log answer within 5 s and 4 MiB. Past either limit the read is cut off, not waited on. A per-read timeout alone never ends a body that keeps trickling in. Container logs are read since each container's cursor, but only its newest 2,000 lines, all containers within 10 s. A container that writes more than that gets a `_steward` note saying older lines were skipped. A container whose answer is too big skips ahead to now. Before this, a 1.13.0 agent re-read a growing backlog every interval and went silent for a day at 2.1 GB. Each sample also carries the agent's own `agent` footprint (`rss_bytes`, `cpu_pct`), which Steward records as `agent_rss_bytes` and `agent_cpu_pct`.
+
 ### What happened to the host (1.14.0, #5756)
 
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