Werkdock skeleton: doctor, load, run over the bwrap engine (Go)

The minimal build-capable CLI decided in RFC 0002's outcome: stdlib-only
Go module, one static binary.

- engine: RunSpec behind the Engine interface (RFC 0001); the Bwrap
  engine ports Werkator's hardened invocation — uid-0 mapping, read-only
  rootfs at /, proc/dev/tmp/root before the user binds so binds below
  them land inside, mountpoint pre-creation in the rootfs including file
  mountpoints, and a guard against binds escaping the rootfs. --clearenv
  gives docker-style clean environments (HOME/PATH set explicitly).
- store: images under $WERKDOCK_HOME (default ~/.werkdock), load
  unpacks via the tar CLI into a tmp dir and renames atomically.
- cli: docker-shaped run flags (-v/-e/-w/--rm); refused docker flags
  (-p, --network, --memory, --cpus, --user, -d) fail loudly with the
  reason; exit codes follow docker (125 CLI errors, child code through).
- doctor: port of werkator-build-prerequisites.sh — userns probe with
  the three signals, tar/zstd, free space and group-quota headroom via
  testable df/quota parsers, same PASS/FAIL output.
- tests: argv golden test, mountpoint and escape tests, flag refusals,
  store round trip, doctor parsers — plus real-sandbox integration
  tests that skip where bwrap or userns are unavailable.

Also records in step 21: RFC 0002 levels 2/3 deferred; next goal is
sandbox builds of Werkator, Werkbaum, and Werkdock itself.

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

docs/rfcs/0002-docker-compatible-surface.md

@@ -2,7 +2,7 @@
 
 **Status:**
 - proposed: 2026-09-01
-- accepted: -
+- accepted: 2026-09-01 (level 1 as the shape of the CLI; levels 2 and 3 deferred indefinitely)
 - rejected: -
 
 **Proposal:** Werkdock's user-facing surface follows Docker wherever the filesystem-only contract allows: level 1 is a docker-compatible CLI (verbs, flags, exit codes), level 2 is pulling OCI images from registries, level 3 is a daemon offering the Docker Engine REST API subset that Testcontainers needs.
@@ -85,4 +85,6 @@ Ryuk stays disabled in documentation until proven.
 
 ## Decision Outcome
 
-Pending.
+Decided 2026-09-01: level 1 shapes the CLI — verbs and flags follow docker, unsupported flags fail loudly.
+Levels 2 and 3 (OCI pull, daemon API, Testcontainers) are deferred indefinitely; nothing in the code may make them harder, nothing is built for them now.
+The immediate goal is narrower than level 1's full verb list: `doctor`, `load`, and `run` — enough for the sandbox builds of Werkator, Werkbaum, and Werkdock itself; the remaining verbs follow with need.