fix(deploy-benchmark): add --subset flag for tau2 domain selection

[vz-ibm]

Problem

lib/sparc-run.sh and lib/baseline-run.sh (in the downstream rosso-sparc
integration scripts) call deploy-benchmark.sh ... --subset "$SUBSET" to
select a tau2 domain (retail/airline/mock/telecom), but deploy-benchmark.sh
has never actually supported that flag:

Error: Unknown option: --subset

Every call silently failed the entire deploy, or was worked around by never
passing --subset at all (defaulting to whatever domain the image was baked
with — normally retail), which made running the airline domain (or any
non-default domain) impossible via this script.

Fix

Adds --subset NAME as a proper CLI flag. It's wired as a bare SUBSET env
var on the deployed container (not through the existing EXGENTIC_SET_*
--set-style mechanism), matching how exgentic-mcp-tau2's own
entrypoint.sh reads it directly (--subset $SUBSET) — that support landed
in Exgentic/exgentic feature/mcp-command commit ae4f1c9.

Requires an exgentic-mcp-tau2 image built from that commit or later; older
images silently ignore the env var and just run their build-time default
domain.

Verification

Ran end-to-end on a live cluster: rebuilt the tau2 MCP image from current
feature/mcp-command, deployed with --subset airline, confirmed via pod
logs:

Command: exgentic mcp --benchmark tau2 --host 0.0.0.0 --port 8000 --subset airline ...
✓ Loaded benchmark 'tau2' with 50 tasks

then ran the full 50-task airline baseline through to completion:
Sessions Attempted: 50, Sessions Succeeded: 50, Sessions With Error: 0.

[vz-ibm]

These PR blocked by xGentic PR#187