Step 21 session A: close step 17's paperwork

- PR-docs get their real numbers: bwrap-build-runtime -> PR#4,
  build-current-head-from-the-branches-view -> PR#3 (files and scenario IDs)
- tools/remote: header note marking install's clone step and build as the
  self-build prototype, superseded by session D of step 21 (usage range grown)
- ADR 0008: bubblewrap user-namespace sandbox as the third build runtime;
  step 17 and the plan README now point at 0008 (0007 was taken by build
  definitions before the step landed)
- AGENTS.md: decisions list catches up with ADR 0007 and ADR 0008
- architecture skill: BwrapBuildRunner paragraph (rootfs unpack, uid mapping,
  mount ordering, pinned keys) and the dispatcher's three-way routing
- plan README: step 21 entry now describes the werkdock/ subdirectory path

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.claude/skills/architecture/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: architecture
-description: Detailed Werkator subsystem architecture — CLI wiring and exit codes, server mode, web UI, configuration system, git access, build execution (native and Docker), watcher poll cycle, and system metrics. Use when designing or modifying code in the commands, config, git, gitea, build, artifacts, watcher, metrics, or server packages, or when a question goes beyond the overview in AGENTS.md.
+description: Detailed Werkator subsystem architecture — CLI wiring and exit codes, server mode, web UI, configuration system, git access, build execution (native, Docker, and bwrap), watcher poll cycle, and system metrics. Use when designing or modifying code in the commands, config, git, gitea, build, artifacts, watcher, metrics, or server packages, or when a question goes beyond the overview in AGENTS.md.
 ---
 
 # Werkator Architecture
@@ -65,7 +65,9 @@ Three places must stay in sync when config keys change: the `WerkatorConfig` dat
 
 On context close (e.g. systemd SIGTERM), a `ContextClosedEvent` listener in `BuildExecutor` terminates the process trees of all executing builds and waits (bounded) until their results are persisted as INTERRUPTED — a shutdown is never recorded as FAILED. Builds still queued stay PENDING and start no process. Both are re-enqueued by the watcher's startup recovery; INTERRUPTED therefore publishes as Gitea state `pending`, not `failure` (`GiteaStateMapping`).
 
-The runtime is selected per branch behind the `BuildRunner` interface: `DispatchingBuildRunner` (`@Primary`) routes to native `ProcessBuildRunner` (the default) or to `DockerBuildRunner` when `branches.<name>.docker.enabled`. The Docker runner shells out to the `docker` CLI (no SDK): it (re)builds the configured image when the Dockerfile inputs changed (tracked via the `org.werkator.build-inputs-sha256` image label), maintains a per-repo Gradle cache volume, mounts the worktree and the Docker socket into a labelled (`org.hoennig.werkator`) `--rm --init` container, and repairs workspace ownership in-container after each command (under a rootless daemon the container runs as root, which is the host user, and the repair degenerates to `0:0`). Git works inside the container: the primary `.git` is mounted read-only with `.git/werkator/` masked by an empty tmpfs (credential isolation) and the worktree's admin dir mounted read-write (`gitMetadataMounts`). The returned `Process` is the attached `docker run` client, so log streaming and termination work exactly like native builds.
+The runtime is selected per build behind the `BuildRunner` interface: `DispatchingBuildRunner` (`@Primary`) routes to native `ProcessBuildRunner` (the default), to `DockerBuildRunner` when `docker.enabled`, or to `BwrapBuildRunner` when `bwrap.enabled` — docker and bwrap are mutually exclusive per build and rejected in `buildSettings`, never picked silently. The Docker runner shells out to the `docker` CLI (no SDK): it (re)builds the configured image when the Dockerfile inputs changed (tracked via the `org.werkator.build-inputs-sha256` image label), maintains a per-repo Gradle cache volume, mounts the worktree and the Docker socket into a labelled (`org.hoennig.werkator`) `--rm --init` container, and repairs workspace ownership in-container after each command (under a rootless daemon the container runs as root, which is the host user, and the repair degenerates to `0:0`). Git works inside the container: the primary `.git` is mounted read-only with `.git/werkator/` masked by an empty tmpfs (credential isolation) and the worktree's admin dir mounted read-write (`gitMetadataMounts`). The returned `Process` is the attached `docker run` client, so log streaming and termination work exactly like native builds.
+
+`BwrapBuildRunner` (ADR 0008) is the third runtime, for hosts without root and without Docker — Hostsharing Managed Webspaces. It shells out to the `bwrap` CLI (no library): a prepared rootfs archive (`bwrap.rootfs`, built by `tools/build-bwrap-rootfs.sh`) is unpacked on demand into `.git/werkator/buildenv/<envKey>/rootfs` and bound read-only at `/`, with uid 0 inside mapped to the calling user; isolation is filesystem-only — network, uid, `/proc`, `/dev` are the host's by contract. It reuses the Docker runner's `gitMetadataMounts`; mount order matters (repo dir read-write before the metadata mounts and the workspace), and bind mountpoints missing from the rootfs are pre-created there, since the rootfs is a plain host directory while bwrap cannot mkdir against the read-only sandbox root. `bwrap.enabled`/`bwrap.rootfs` are pinned like the docker sandbox policy. The returned `Process` is the attached `bwrap` process, so streaming and cancellation are unchanged. Plan step 21 will extract the generic sandbox machinery into the standalone tool Werkdock (grown in `werkdock/`); the runner then delegates to the `werkdock` CLI.
 
 ## Watcher