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  self-build prototype, superseded by session D of step 21 (usage range grown)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>

docs/plan/17-bwrap-build-runtime.md

@@ -168,7 +168,7 @@ Two claims could **not** be verified from a Hostsharing primary source; check th
 
 ## ADR
 
-Write ADR 0007: bubblewrap user-namespace sandbox as the third build runtime (options considered: bwrap (chosen), proot/fakechroot (slow, fragile), plain native with hand-installed toolchains (no isolation, host pollution)).
+Write ADR 0008 (step text originally said 0007, but 0007 was taken by build definitions): bubblewrap user-namespace sandbox as the third build runtime (options considered: bwrap (chosen), proot/fakechroot (slow, fragile), plain native with hand-installed toolchains (no isolation, host pollution)).
 
 ## Tests
 
@@ -182,4 +182,4 @@ Write ADR 0007: bubblewrap user-namespace sandbox as the third build runtime (op
 - `./gradlew ktlintFormat` then `./gradlew build` is green — also on a machine without Docker (Testcontainers smoke test skipped, not failed).
 - On a Managed Webspace: Werkator (from the runtime bundle) builds a real branch of a repo inside the bwrap sandbox; git commands work in the worktree; `.git/werkator/` is not readable from the build; a write to `/usr` fails.
 - On the same webspace: the UI answers over HTTPS under the domain through the Apache `.htaccess` proxy, the service survives a logout and a reboot (systemd lingering), and Gitea statuses carry `publicBaseUrl` links that resolve.
-- Docs updated: `docs/configuration.md` (bwrap section), architecture skill (third runtime), ADR 0007, and `docs/deployment.md` gains "Hostsharing Managed Webspace" as a third deployment variant — written only once the setup above is verified on a real webspace, not from this plan.
+- Docs updated: `docs/configuration.md` (bwrap section), architecture skill (third runtime), ADR 0008, and `docs/deployment.md` gains "Hostsharing Managed Webspace" as a third deployment variant — written only once the setup above is verified on a real webspace, not from this plan.

docs/plan/README.md

@@ -93,7 +93,7 @@ Added for running Werkator on Hostsharing Managed Webspaces (2026-08-10):
 
 Added to correct the bwrap prototype's drift toward self-building on the webspace (2026-09-01):
 
-- [ ] `21-werkdock-extraction-and-webspace-install.md` — roadmap in four sessions: close step 17's open ends, extract the sandbox tooling into a new repository (**Werkdock**, a docker-like filesystem-only sandbox CLI), let Werkator consume it, and replace the webspace self-build with the local-build-plus-install path of ADR 0006
+- [ ] `21-werkdock-extraction-and-webspace-install.md` — roadmap in four sessions: close step 17's open ends, grow the sandbox tooling into **Werkdock** (a docker-like filesystem-only sandbox CLI, developed in the `werkdock/` subdirectory, later its own repository), let Werkator consume it, and replace the webspace self-build with the local-build-plus-install path of ADR 0006
 
 Added for surfacing build time as a trend (2026-08-31):
 
@@ -105,7 +105,7 @@ Steps 07–09 depend on 04–06.
 Steps 11 and 12 are optional/deferrable; 10 only needs 04–06.
 Step 13 depends on 07, 11, and 12.
 Step 15 depends on 12 and 13 and revises the containerized-runtime sketch in `docs/bootstrapping.md` (ADR 0006 is written as part of the step; GraalVM native image was evaluated and rejected there).
-Step 17 depends on 11, 15, and 16, and starts with a hard precondition check on the target webspace (ADR 0007 is written as part of the step).
+Step 17 depends on 11, 15, and 16, and starts with a hard precondition check on the target webspace (ADR 0008 is written as part of the step; the number 0007 announced in the step file was already taken).
 Step 18 depends on nothing in code but on the watched repository having migrated — its precondition check is a hard gate, not a formality.
 Step 19 depends on nothing; `WatcherState` and `/api/watcher` already carry everything it needs to render.
 Step 20 depends on nothing; the duration is already recorded, and the trend is derived read-only from `repository.history()`.