feat(builder): every boot through the node-agent, every lost file counted, Steam cache

What a builder boot sees is now what a node does:
- First boots (untouched, or -as-deployed with exactly a new server's
  settings) are deployed by the node-agent's BootTest in keep mode; the
  builder only watches the console and inspects what's left. The Go copy of
  the deploy (its own docker run, mounts and startup prefix) is gone. It had
  drifted: it gave the image's VOLUME folders their own volumes, which nodes
  don't, so saves there looked safe while every start loses them.
- Files written outside /server and /data are counted whatever their type
  (it only counted config-like extensions, so .sav or .db saves went
  unseen), including the image's VOLUME folders; logs, pid and lock files
  and system folders are left out.
- Boot tests ask the server to stay up holdSec (60 s, test.yaml can set it)
  after its ready line and to listen on every published container port
  (needs the node-agent's matching BootTest).
- Steam apps are downloaded once per build into a gbr.cache volume and copied
  into each boot, so later boots only check the files (STEAM_CACHE=off).
- RUNNER.md no longer suggests docker system prune, which deletes the
  gbr-boottest network when nothing is using it.

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

BUILDER.md

@@ -19,8 +19,18 @@ progress / log / variants        ◄─post── build: first boot → draft
 2. The admin picks a variant in the panel.
 3. **Build** (`gbrgames build -name N -variant @v.json`): first boot of the image untouched (`first-boot`), a
    draft by the model (`draft`, rules in `AUTHORING.md`), then the draft is deployed with the node-agent's
-   `BootTest`, the same check CI runs. Failures (crash, error lines, saves written outside `/server`) go
-   back to the model, three boots at most.
+   `BootTest`, the same check CI runs. A failed boot is investigated before the model changes anything
+   (see below).
+
+   **Every** server the builder starts, first boots included, is deployed by the node-agent's own code, so
+   it runs exactly as on a node. A boot passes when the server reaches its ready line, stays up for
+   `holdSec` (60 s) after it, and listens on every port the template publishes. Files the server writes
+   anywhere but `/server` and `/data` (the image's own `VOLUME` folders included) are reported as lost:
+   nodes recreate the container on every start.
+
+   Steam games are downloaded once per build into a cache volume (`gbr-steamcache-<app>`, labelled
+   `gbr.cache`) and copied into each boot, so later boots only check the files. `STEAM_CACHE=off` turns
+   it off.
 4. The worker pushes `games/<id>/` to a branch and opens a pull request as the bot account. A draft that
    validates but never booted still gets a PR, titled `WIP:`, with what's left listed. Nothing reaches
    players until a person merges it and enables the game.