Gate the build on Docker lint and test phases (closes #40, closes #30)

Per the owner ruling on issue 40, linting and testing are phases of the
main Dockerfile rather than a separate lint file. script/lint and
script/test each build one phase by name with caching disabled, and the
final stage copies a harmless file from each so the image cannot be built
unless both passed — the ordering trick already in template-app-go's
Dockerfile, extended to the test phase. A phase that is not the last
stage is built only when something depends on it or --target names it, so
the gates are always invoked by name and the edges are kept. Every build
in script/ is tagged. No config verify step; fmt stays on the host. This
closes issue 30 too: a container has its own result cache and lock.

Model: opus-5

TODO.md

@@ -21,6 +21,14 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-09-08: Moved linting and testing into Docker as phases of the main
+  `Dockerfile`, per the owner ruling on issue 40. `script/lint` and
+  `script/test` build one phase each by name with `--no-cache` — the same answer
+  issue 26 got, so no separate cache-busting mechanism survives — and the final
+  stage copies a harmless file from both, so the image cannot be built unless
+  they pass. This also closes issue 30: a container has its own result cache and
+  its own lock, so a lint verdict can no longer belong to another checkout. No
+  separate lint Dockerfile, and no `golangci-lint config verify` step.
 - 2026-09-08: Kept in-repo agent scratch out of the Docker build context and out
   of version control: `.claude/` is one full checkout of the repo per in-flight
   agent, and under `COPY . .` all of it was reaching the image. Also closed the