fix(docker): bake build-time git SHA into the image

`hermes dump` and the startup banner both call `git rev-parse HEAD` to
report the running commit, but `.dockerignore` line 2 excludes `.git` —
so inside the published image `hermes dump` shows
`version: ... [(unknown)]` and the banner drops its `· upstream <sha>`
suffix entirely.  That makes support triage from container bug reports
impossible: we can't tell which commit the user is actually running.

Fix: thread the build-time SHA through as a Docker build-arg, write it
to `/opt/hermes/.hermes_build_sha` in the image, and have a new
`hermes_cli/build_info.get_build_sha()` read it as a fallback after the
existing live-git lookup fails.  Output format is unchanged in both
callsites — same 8-char short SHA whether resolved live or baked.

Wiring:
  - Dockerfile: `ARG HERMES_GIT_SHA=` + write-file step after the source
    copy.  Empty/missing arg → no file written → callers fall through to
    live git (so local `docker build` without --build-arg is unchanged).
  - docker-publish.yml: passes `HERMES_GIT_SHA=${{ github.sha }}` on all
    four build-push-action steps (amd64/arm64, smoke-test + final push).
  - dump.py:_get_git_commit() / banner.py:get_git_banner_state(): try
    live git first, fall back to baked SHA, then to legacy `(unknown)`
    / None.  Banner returns `upstream == local, ahead=0` because a built
    image is by definition pinned to one commit.

Coverage:
  - Unit tests cover build_info (file present/absent/empty/error,
    truncation, whitespace), dump (live-git wins, both fallbacks,
    identical output-format regression guard), and banner (no-repo +
    baked, no-repo + no-sha, shallow-clone fallback).
  - tests/docker/test_dump_build_sha.py is an integration regression
    guard that runs against the real image, reads
    `/opt/hermes/.hermes_build_sha`, and asserts `hermes dump` surfaces
    its content (or stays at `(unknown)` if no file).
  - Verified end-to-end: `docker build --build-arg HERMES_GIT_SHA=abc...`
    → `docker run ... dump` reports `[abc12345]`; without the build-arg
    it reports `[(unknown)]` as before.

.github/workflows/docker-publish.yml

@@ -71,6 +71,8 @@ jobs:
           load: true
           platforms: linux/amd64
           tags: ${{ env.IMAGE_NAME }}:test
+          build-args: |
+            HERMES_GIT_SHA=${{ github.sha }}
           cache-from: type=gha,scope=docker-amd64
           cache-to: type=gha,mode=max,scope=docker-amd64
 
@@ -149,6 +151,8 @@ jobs:
           platforms: linux/amd64
           labels: |
             org.opencontainers.image.revision=${{ github.sha }}
+          build-args: |
+            HERMES_GIT_SHA=${{ github.sha }}
           outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest=true,name-canonical=true,push=true
           cache-from: type=gha,scope=docker-amd64
           cache-to: type=gha,mode=max,scope=docker-amd64
@@ -203,6 +207,8 @@ jobs:
           load: true
           platforms: linux/arm64
           tags: ${{ env.IMAGE_NAME }}:test
+          build-args: |
+            HERMES_GIT_SHA=${{ github.sha }}
           cache-from: type=gha,scope=docker-arm64
           cache-to: type=gha,mode=max,scope=docker-arm64
 
@@ -228,6 +234,8 @@ jobs:
           platforms: linux/arm64
           labels: |
             org.opencontainers.image.revision=${{ github.sha }}
+          build-args: |
+            HERMES_GIT_SHA=${{ github.sha }}
           outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest=true,name-canonical=true,push=true
           cache-from: type=gha,scope=docker-arm64
           cache-to: type=gha,mode=max,scope=docker-arm64