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.

tests/hermes_cli/test_banner_git_state.py

@@ -61,3 +61,56 @@ def test_get_git_banner_state_reads_origin_and_head(tmp_path):
         state = banner.get_git_banner_state(repo_dir)
 
     assert state == {"upstream": "b2f477a3", "local": "af8aad31", "ahead": 3}
+
+
+def test_get_git_banner_state_falls_back_to_build_sha_when_no_repo():
+    """Docker image case: no .git checkout — baked build SHA fills the gap.
+
+    ``_resolve_repo_dir`` returns None when neither the running code's
+    parent nor ``$HERMES_HOME/hermes-agent/`` is a git repo (the canonical
+    case inside the published container, where .git is dockerignored).
+    The banner should still report the build SHA so support bug reports
+    can identify the running commit.
+    """
+    from hermes_cli import banner
+
+    with patch.object(banner, "_resolve_repo_dir", return_value=None), \
+         patch("hermes_cli.build_info.get_build_sha", return_value="abcdef12"):
+        state = banner.get_git_banner_state()
+
+    assert state == {"upstream": "abcdef12", "local": "abcdef12", "ahead": 0}
+
+
+def test_get_git_banner_state_returns_none_when_no_repo_and_no_build_sha():
+    """Pip-installed wheel with neither git checkout nor baked SHA → None.
+
+    Banner correctly omits the upstream/local suffix in this case.
+    """
+    from hermes_cli import banner
+
+    with patch.object(banner, "_resolve_repo_dir", return_value=None), \
+         patch("hermes_cli.build_info.get_build_sha", return_value=None):
+        state = banner.get_git_banner_state()
+
+    assert state is None
+
+
+def test_get_git_banner_state_falls_back_when_live_git_returns_nothing(tmp_path):
+    """Shallow clone without origin/main → still surface build SHA if baked.
+
+    Some install paths (e.g. ``git clone --depth 1`` without a remote) have
+    a ``.git`` directory but ``git rev-parse origin/main`` fails.  When that
+    happens AND a baked SHA exists, return the baked one instead of None.
+    """
+    from hermes_cli import banner
+
+    repo_dir = tmp_path / "repo"
+    (repo_dir / ".git").mkdir(parents=True)
+
+    # All git invocations fail (returncode=1, empty stdout).
+    failed = MagicMock(returncode=1, stdout="")
+    with patch("hermes_cli.banner.subprocess.run", return_value=failed), \
+         patch("hermes_cli.build_info.get_build_sha", return_value="cafef00d"):
+        state = banner.get_git_banner_state(repo_dir)
+
+    assert state == {"upstream": "cafef00d", "local": "cafef00d", "ahead": 0}