[techdigger]

Stacked on #1, so the diff shows only this PR's two commits. Once #1 is merged and its branch deleted, GitHub retargets this PR to main.

What

1. Chain 5234 default. On Humanode, --legacy-gas-price is on without the flag, so owners cannot forget it. SAFE_CLI_LEGACY_GAS=0 turns it off, and --legacy-gas-price=<gwei> still sets a price by hand. This commit stays in our fork only and is not proposed upstream.
2. CI (.github/workflows/humanode-build.yml), on every pull request and on pushes to main:
   • runs upstream's test suite against Ganache, the way upstream's ci.yml does;
   • builds the wheel that owners install, with a SHA-256 manifest.

Tested

• New unit test test_default_on_humanode: on by default on chain 5234, off with SAFE_CLI_LEGACY_GAS=0, off on other chains.
• The same workflow ran green on this code before the PR, when it was triggered by a push: https://github.com/humanode-network/safe-cli/actions/runs/43254582539

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