docs(infra): host telemetry rules out disk/memory/PID exhaustion and fail2ban

Server shows 62G free (71% used), inodes 14%, 16G /dev/shm (~32G RAM), ulimit
-u 127749, no cgroup pids.max, and no fail2ban installed. That falsifies both
resource-exhaustion hypotheses at the host level and the fail2ban explanation
for the deploy failure.

Notes the caveat that Gitea act_runner usually runs jobs inside a Docker
container, so host figures do not describe the environment the tests ran in
(separate cgroup limits, and a 64MB /dev/shm by Docker default).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/infrastructure/runner-investigation.md

@@ -76,6 +76,33 @@ in order of likelihood:
    extraction, corrupted downloads, and silent process deaths are all classic disk-full signatures.
    `testhost` writes `TestResults/` and may write dumps.
 
+### Host telemetry (2026-07-18, post-reboot) — resource exhaustion ruled out
+
+Collected from the server:
+
+```
+/            217G  146G  62G  71%      (inodes 14% used)
+/dev/shm      16G     0   16G   0%     (=> ~32 GB RAM)
+ulimit -u  127749   ulimit -n 1024   ulimit -m unlimited
+/sys/fs/cgroup/pids.max: No such file or directory
+fail2ban-client: command not found
+journalctl -u sshd: No entries      (Ubuntu's unit is `ssh`, not `sshd`)
+dmesg: read kernel buffer failed: Operation not permitted   (needs sudo; cleared by reboot anyway)
+```
+
+**This falsifies both resource hypotheses at the host level**: there is no disk pressure, no inode
+pressure, no memory pressure and no restrictive PID limit. It also falsifies the fail2ban explanation
+for the deploy failure — fail2ban is not installed.
+
+**Caveat that matters:** Gitea `act_runner` normally executes a `runs-on: ubuntu-latest` job **inside
+a Docker container**, so the figures above describe the *host*, not the environment the tests
+actually ran in. A job container has its own cgroup memory/PID limits and, by Docker default, a
+**64 MB `/dev/shm`** regardless of the host's 16 GB. The relevant limits have therefore not been
+measured yet — see the asks below.
+
+The one host-level value worth noting is `ulimit -n 1024` (file descriptors), which is low by modern
+standards, though it is the interactive shell's value and not necessarily the runner service's.
+
 ### Confidence level
 
 - **Application code is not the cause — high confidence (~95 %).** Ten independent environments,