Refuse build steps under a free-space floor and release workspaces, so the build modules stop filling a build node's disk (issue 1839)

mesh-build-go, mesh-build-node and mesh-build-image refuse every fetch, test
and build, and say unhealthy, while their work or cache filesystem has less
than 20 GiB free (build-agent's floor), so the builder chooses another node
instead of filling this one. Each serves build_<x>_release, removing a
workspace once its caller's last step answered; a failed fetch removes its
own; a workspace never released goes two hours after its last use (ADR 0509)
in place of a day, never while a step holds it. Pins mesh-sdk's
fix/1839-build-floor (buildstep.Floor, buildstep.Workspaces).

modules/mesh-build-node/README.md

@@ -57,7 +57,11 @@ toolchain install: a long step answers at once with its id and ends on `<node>.<
   removed). The mesh's registry reaches `npm ci` through a private config file (`--userconfig`), so
   other `@novox/` packages install. A module with packages and no lock does not fail the fetch: its result lists it
   under `refused` with the reason, and a build of that bundle fails naming it (issue 1459). Every command runs through `forgekey.ExecRunnerClean(0)` with PATH, HOME, the locale and its own
-  words, nothing of this process's environment. Workspaces older than a day are removed at each fetch.
+  words, nothing of this process's environment. A failed fetch removes its own workspace; `build_node_release {workspace}` removes one at once (no guard, no
+  floor; its caller calls it once its last step answered), and one no caller released is removed two hours after its
+  last use (ADR 0509), never while a step holds it. Every step is refused, `refused: <node> has N GiB free under the
+  floor of 20 GiB (on <dir>)`, and the health is unhealthy with that text, while the work or the toolchain cache
+  filesystem has less than 20 GiB free (issue 1839).
   The arguments are mesh-sdk's `buildstep.FetchRequest` (ADR 0499 §4): with `dir` and `paths` (from the builder, never
   from a manifest) the workspace is narrowed to those paths, the manifest and the lock files before anything else. A
   member is built against the SDK version its own lock names (the lead's decision L10 of R4e, ADR 0500 §5-§6): the