cli: read Hive aggregate subnet env

[zclawz]

Summary

• accept HIVE_AGGREGATE_SUBNET_IDS as a startup fallback for aggregator subnet selection
• keep explicit --aggregate-subnet-ids behavior and validation unchanged
• factor aggregate subnet CSV parsing into a helper with unit coverage

Hive context

Suite: https://hive.leanroadmap.org/suite.html?suiteid=1784586818-a3eb27685f4418cb92cbcec02266f5f5.json&suitename=client-interop&client=zeam_devnet5

The client-interop simulator sets HIVE_AGGREGATE_SUBNET_IDS for multi-subnet aggregators, and gean/grandine forward that value as --aggregate-subnet-ids. The zeam Hive wrapper currently only forwards --is-aggregator, which leaves zeam aggregators subscribed to their local validator subnets instead of all assigned aggregate subnets. This makes zeam miss cross-subnet attestation traffic in the failing interop topologies.

Reading the env var in zeam keeps the existing CLI flag as the operator-visible override while making the devnet5 Hive client behave with the simulator's aggregator assignment contract.

Validation

• zig fmt --check pkgs/cli/src/node.zig
• git diff --check
• /home/node/.openclaw/workspace/.tools/zig-0.16.0/zig build test --summary all was started and reached/passed several lightweight package test groups, but the local container cannot complete the repo test target because rustup is not installed (failed to spawn ... rustup: FileNotFound).

Note: I used Zig 0.16.0 from /home/node/.openclaw/workspace/.tools/zig-0.16.0/zig; the default /usr/local/bin/zig is 0.15.2 and cannot load this checkout's dependencies.

[zclawz]

Status: ready for review.

I traced the zeam-owned Hive interop issue to aggregator subnet selection: Hive sets HIVE_AGGREGATE_SUBNET_IDS for multi-subnet aggregators, but zeam was only reading the CLI flag. This PR reads that env var as a fallback when --is-aggregator is set, preserving explicit CLI behavior.

Validation so far:

• zig fmt --check pkgs/cli/src/node.zig
• git diff --check
• started zig build test --summary all with Zig 0.16.0; local run is blocked by missing rustup, after several lightweight Zig package test groups passed.

[ch4r10t33r]

LGTM