docs(host-agent): document container-log collection [M79 step 6]

Design-spec updates for m79: the docker_logs_enabled / docker_log_exclude config
keys, the collect_docker_logs collector + the drop-logs-on-backoff note in the
agent internals, and a wire-format design point on docker_logs riding in the same
push (with server-side toggle/exclude enforcement + the per-container ring).

The end-to-end chain (store → rotate → query + ingest enforcement) is already
covered by the integration tests added across steps 2–5, so no capstone test is
added. AGENT_VERSION bump (1.7.0) landed in step 1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C

docs/plugins/host-agent-design.md

@@ -74,8 +74,15 @@ token = a1b2c3d4...
 interval_seconds = 30
 hostname = myhost              # optional; defaults to uname -n
 mounts = /, /mnt/data          # optional; defaults to all real mounts (excluding tmpfs/devtmpfs/etc.)
+docker_logs_enabled = true     # optional; on by default — per-host container-log kill-switch
+docker_log_exclude = watchtower, noisy-svc   # optional; container names whose logs this host skips collecting
 ```
 
+Container-log collection is on by default (no config needed). The two keys above
+are the per-host opt-outs; the operator-facing global toggle + exclude live in
+Settings and are enforced server-side (the push model has no channel to command
+an agent), so they don't require touching a host's conf.
+
 ### Agent internals (function list, not classes)
 
 - `read_config(path)` — parses the conf file into a dict.
@@ -85,10 +92,17 @@ mounts = /, /mnt/data          # optional; defaults to all real mounts (excludin
 - `collect_load()` — reads `/proc/loadavg`, returns `[1m, 5m, 15m]`.
 - `collect_uptime()` — reads `/proc/uptime`, returns seconds since boot (int).
 - `collect_metadata()` — `os.uname()` for kernel + arch, `/etc/os-release` for distro. Called once at startup and cached.
+- `collect_docker_logs(socket, containers, state, exclude)` — (m79) per running
+  container, fetches new log lines over the Docker socket with an incremental
+  `since` cursor kept per container in `state` (a container's first interval
+  seeds from a short tail). Demuxes the multiplexed stream, parses each line's
+  RFC3339 timestamp, caps the whole batch at a byte limit (a marker line records
+  a truncation; the deferred lines come next interval). Folded into the sample as
+  `docker_logs`; omitted when empty.
 - `build_payload()` — assembles a snapshot from all collectors into one dict.
 - `post_payload(url, token, payloads)` — POSTs a list of samples, returns success/failure.
 - `RingBuffer(maxlen=20)` — tiny FIFO wrapper, drops oldest when full.
-- `main_loop()` — the 30s loop: collect → try POST → on failure push to buffer + backoff → on success flush buffer.
+- `main_loop()` — the 30s loop: collect → try POST → on failure push to buffer + backoff → on success flush buffer. Container logs are stripped from a sample before it's buffered (metrics survive an outage; stale logs are dropped).
 
 **Target: ~300 lines total including docstrings.** More than that is a smell that the agent is over-scoping.
 
@@ -255,6 +269,7 @@ Content-Type: application/json
 - **`metadata` is sent on every POST**, not just on change. Server-side diff detects actual changes and only writes on change. Cost per POST is one dict — negligible. Benefit: server can cleanly detect agent restarts.
 - **Raw bytes, not percentages, for memory and storage.** Percentages are derived server-side. Changing the "what counts as used" math doesn't require re-releasing the agent.
 - **CPU is the one exception** — reported as a percentage because it's inherently a derivative (delta over time), not a snapshot. The agent must sample twice to compute it.
+- **Container logs (m79) ride in the same push** as `docker_logs`: a list of `{container, stream, ts, line}` records — incremental since the previous interval. This pushes the *same direction* as metrics, so batched log history needs no inbound channel (only sub-second live-follow would). The server ingests them into `docker_logs`, enforces the global toggle + exclude list on ingest, and bounds storage with a per-container size+age ring. `docker_logs` is omitted when there's nothing new.
 
 ### Server expansion into `PluginMetric` rows