feat: add SIGNET_DISABLE_EXEX env var for vanilla reth mode

[rswanson]

Summary

• When SIGNET_DISABLE_EXEX=true (or 1), the node runs as vanilla reth without the Signet ExEx
• Enables initial chain sync to complete using a single signet binary instead of requiring a separate reth image
• No change to default behavior — without the env var, signet runs normally with ExEx

Context

This is a prerequisite for init4tech/the-infra to simplify the init-sync architecture. Instead of running a separate reth container as a sidecar (which caused crashloop and resource waste issues), the infra wrapper script can now run signet itself in vanilla reth mode for sync, then restart it with ExEx enabled.

Test plan

• cargo fmt --check passes
• cargo clippy passes
• Verify node starts normally without SIGNET_DISABLE_EXEX (existing behavior unchanged)
• Verify node starts as vanilla reth with SIGNET_DISABLE_EXEX=true
• Verify SignetNodeConfig env vars are not required when ExEx is disabled

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[rswanson]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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