fix: empty zeam_global_flags default for devnet3 image

[ch4r10t33r]

Problem

Merged #147 added --console-log-level debug before node. The published blockblaz/zeam:devnet3 image is built from an older Zeam that does not define that top-level CLI option, so the node exits with:
Unknown option '--console-log-level'.

Fix

• Default no global flags in zeam-cmd.sh; opt in with ZEAM_GLOBAL_FLAGS when using a Zeam binary that supports them (e.g. local zig-out/bin/zeam).
• Ansible: zeam_global_flags: "" by default; override in inventory when the image supports log flags.

Verification (reasoning)

• Empty zeam_global_flags in bash: zeam $zeam_global_flags node word-splits to zeam then node (no unknown argv).
• Ansible: empty Jinja expands to nothing; docker run ... IMAGE node remains valid (extra whitespace between args is fine).
• Confirmed main still had hard-coded --console-log-level debug after client-cmds: default zeam console log level to debug #147 squash; this commit matches the follow-up that was on zeam-default-console-debug but not in the merge.