[P2] CI: runtime-server changes select zero downstream harness image builds

[reacher-z]

Problem

check-harness-build triggers for src/clawbench/runtime/**, but its dependency selector marks all harnesses for extension-server/ changes, not the current runtime-server/ directory. A change only to runtime-server code or its dependency lockfile builds the base image but selects zero downstream harness builds.

Sources: selection case statement, base image copies runtime-server.

Verified selection reproduction

Executed the workflow's actual for path ... case ... esac block in Bash with:

changed_runtime_files=(src/clawbench/runtime/runtime-server/server.py)
build_all=false
changed_dockerfiles=()

Result: build_all=false selected_harnesses=0. The same path-category mismatch applies to runtime-server/pyproject.toml and runtime-server/uv.lock.

The separate build-base job still runs; this finding is specifically the missing downstream coverage, not a claim that no build occurs. No Docker build was run locally.

Acceptance criteria

• Recognize runtime-server/ as a shared base dependency and select all affected harnesses.
• Remove or document the obsolete extension-server/ selector.
• Extract/test the image-selection logic with representative runtime-server, base, extension, shared, single-harness and irrelevant changes.
• Confirm workflow summaries show the correct affected matrix for runtime-server-only changes.

Suggested priority P2: close a blind spot in an existing quality gate. This does not request Harbor E2E CI or reopen the deferred #350; it repairs the native harness build workflow already in use.

@Perry2004 please review the dependency-selection matrix and expected native harness coverage.