Cut v0.75.5b — fix cache-hit silent same-bytes regression

ALL FOUR releases v0.74.0 -> v0.75.5 had been shipping the same image
bytes due to a Docker layer-cache hit on the bare 'npm install -g
@earendil-works/pi-coding-agent' command (when PI_VERSION=latest).
The command string is identical across builds, so the layer-hash is
identical, so registry buildcache (cache-from/cache-to) silently
reuses the layer from whatever pi version was current when the cache
was first populated.

Verification: docker manifest inspect joakimp/pi-devbox:vX.Y.Z showed
identical SHA256 digests on both linux/amd64 and linux/arm64 for
v0.74.0, v0.75.3, v0.75.4, v0.75.5. Users on :latest were getting
whatever pi version was baked into the v0.74.0 build.

DISCOVERED 2026-05-23 by user trying to update pi-devbox on MBP-M1
and seeing pi 0.74.0 reported despite pulling v0.75.5.

CHANGES

.gitea/workflows/docker-publish.yml — both smoke and publish jobs
get a new 'Resolve PI_VERSION from tag' step that strips the leading
'v' and any trailing letter suffix from github.ref_name. Result is
passed as a build-arg to docker/build-push-action so the npm install
layer's hash includes the concrete version, forcing cache miss when
pi bumps.

scripts/smoke-test.sh — new run_expect helper that asserts pi
--version contains the EXPECTED_PI_VERSION env var. Smoke job sets
this from the resolve step output. Would have caught this regression
on v0.75.3.

Dockerfile — comment block above ARG PI_VERSION=latest documenting
the cache-hit footgun. The 'if latest' branch in the install RUN is
preserved for local dev convenience but never fires in CI now.

AGENTS.md — new convention bullet explaining the cache-hit class of
bug and noting the latent same-bug in opencode-devbox's with-pi
variants (currently masked by OPENCODE_VERSION bumps; will manifest
when cutting a vN.N.Nb-style opencode-version-unchanged release that
only bumps pi).

CHANGELOG.md — full entry under v0.75.5b describing the recovery,
the silent-failure mechanism, and the verification steps.

NO IMAGE-CONTENT CHANGES vs v0.75.5 INTENT. This build produces the
actual pi 0.75.5 image content that v0.75.5 was supposed to ship.

NEXT FOLLOWUP (parked, not in this commit)

opencode-devbox should get the same workflow change for its
build-variant-with-pi and build-variant-omos-with-pi jobs. Currently
masked because every release also bumps OPENCODE_VERSION which
invalidates the cache, but that masking would fail on a pi-only bump
release.

Dockerfile

@@ -16,6 +16,13 @@
 ARG BASE_IMAGE=joakimp/opencode-devbox:base-latest
 FROM ${BASE_IMAGE}
 
+# PI_VERSION should be passed explicitly by CI as a concrete version
+# (e.g. PI_VERSION=0.75.5, derived from the git tag). The default `latest`
+# is for local dev convenience only — it has a known cache-hit footgun
+# when used in registry-cached CI builds. See .gitea/workflows/docker-
+# publish.yml § "Resolve PI_VERSION from tag" and AGENTS.md gotcha for
+# the full story (silent same-bytes-across-releases regression discovered
+# 2026-05-23 affecting all builds v0.74.0..v0.75.5).
 ARG PI_VERSION=latest
 ARG PI_TOOLKIT_REF=main
 ARG PI_EXTENSIONS_REF=main