fix(docker): drop vendor/tokpress COPY step (build broken since v0.9.2)

[ArkNill]

Summary

Docker workflow has been failing on every tag push since the
vendor/tokpress COPY step was added (sometime between v0.9.1 and
v0.9.2). Both v0.9.4 (PR #22) and v0.9.5 (PR #23) tag pushes failed
identically. This PR removes the broken step.

What was happening

Dockerfile:9
COPY vendor/tokpress /tmp/tokpress
ERROR: failed to compute cache key: failed to calculate checksum
  of ref ...: "/vendor/tokpress": not found

The vendor/ directory is .gitignored (line 15) -- the source has
always been kept outside this repo. The COPY only worked locally
because vendor/ exists on the maintainer's filesystem; on the CI
runner it never existed, so the build failed at step 4/10.

The last successful Docker build was v0.9.1 (2026-04-29), which
predates the tokpress COPY step.

Why removing it is safe

src/llm_relay/proxy/proxy.py already guards the import:

_tokpress_available = False
if _TOKPRESS_ENABLED:
    try:
        from tokpress.integrations.proxy import compress_tool_results
        _tokpress_available = True
    except ImportError:
        pass

And every call site checks _tokpress_available before using the
function. With the package absent the flag stays False and the
optional compression feature is simply disabled -- which is exactly
the runtime behaviour every successfully-built Docker image has had
since the failure was introduced (i.e. none of them, because none
were built).

No published image regresses; the actual regression is that the
last runnable image is v0.9.1 and we want v0.9.5 to be available
on ghcr.io.

Changes

• Dockerfile: drop the COPY + RUN pair; replace with a short
  comment explaining the optional layer is intentionally absent
  from the published image.
• CHANGELOG.md: noted under [Unreleased] ### Fixed. No version
  bump -- this is a CI / image build fix, not a code change visible
  to PyPI consumers.

Test plan

• pytest -- 593 pass (no source changes).
• ruff check src/ tests/ -- clean.
• NDA grep on diff -- 0 hits.

Post-merge

After this lands on main, re-trigger the Docker workflow against
the existing 0.9.5 tag so the image actually publishes:

gh workflow run docker.yml -f tag=0.9.5

That produces ghcr.io/arknill/llm-relay:0.9.5 from the current
main (which has this Dockerfile fix), matching the PyPI 0.9.5
release.