fix(build): make the documented docker build work standalone

[itay-nakash]

README.md and docs/setup.md both tell you to build the image with

docker build -t context-guru:local .

from the repo root. That command cannot work, and this PR makes it work.

go.mod pins bifrost as an ordinary published dependency:

require github.com/maximhq/bifrost/core v1.7.0

There is no replace directive, and docs/setup.md already says so. But the
Dockerfile was still written for the old layout, and not only in comments:

COPY bifrost/ ./bifrost/                    # a directory that need not exist
COPY lab-context-engineering/ ./lab-context-engineering/
WORKDIR /src/lab-context-engineering

So the image could only be built from a PARENT directory holding a sibling
bifrost checkout, while two places in the repo documented the opposite. Run the
documented command today and the first COPY fails. That is the justification for
this change: the two halves of the repo disagreed, and the documented path was
the losing one.

What changes, per file:

• Dockerfile, the only functional change. The build context becomes the repo
  (COPY . .), the WORKDIR /src/lab-context-engineering line goes away, and the
  two runtime-stage COPY lab-context-engineering/deploy/... paths become
  deploy/.... The header comment now documents the same command README does.
• deploy/eval-containers/README.md drops the
  cd .../context-engineering + -f lab-context-engineering/Dockerfile
  incantation for a plain docker build -t context-guru-proxy:latest .. It also
  deletes a "Known gaps (tracked, P5)" bullet that is now factually resolved:
  "The image build needs the parent-dir context (or go mod vendor) until bifrost
  is pinned to a published version." bifrost IS pinned, so the gap no longer
  exists. If that bullet maps to a real P5 ticket, the ticket should be closed
  too.
• examples/llm-d-service/README.md removes the sibling-checkout prerequisite and
  its ASCII directory-layout block, replaced by one line saying the repo builds
  standalone from a plain clone.
• examples/llm-d-service/build.sh is a comment-only change. The go build line
  always worked; the comment just told you to go set up a sibling checkout first.

Net -15 lines. It deletes stale claims and corrects paths; it adds no new
mechanism, no Go code, and nothing that can move a benchmark number.

Verification, both directions executed rather than reasoned about:

new Dockerfile, docker build from the repo root   rc=0, 14/14 steps, 192 MB
OLD Dockerfile, same command                      rc=1, COPY failed:
                                                  stat bifrost/: file does
                                                  not exist
/opt/gateway/{main,start,health} in the image      all three, executable
CGO binary runs on debian:bookworm-slim            yes (no missing libs)
default ENTRYPOINT /opt/gateway/start              runs, demands EVAL_MODEL
                                                   (its own contract)
/opt/gateway/main + GET /healthz                   ok
the image's own /opt/gateway/health                rc=0
make lint / make test / make build                 pass

Also checked against the current tree rather than the tree this was written on:
the dashboard added in #30/#58 ships its UI through //go:embed in dash/ui.go, so
COPY . . covers it and the runtime stage needs nothing extra.

Two things deliberately left out, both reasonable review requests:

• There is still no .dockerignore, so COPY . . sends the whole working tree to
  the daemon (including bin/ and coverage.out if present). Harmless for a
  multi-stage build -- the final image receives only the compiled binary plus the
  two scripts -- but adding one would speed the build. Left out to keep this
  commit to one concern.
• CI does not build the image at all: .github/workflows/ci.yaml runs make lint,
  make cover, make build and a Trivy scan. So nothing in CI would have caught the
  original breakage, and nothing will validate this fix automatically either.