feat(gateway): add config set overrides

[matthewgrossman]

Summary

Add a generic, atomic openshell-gateway config set command and expose repeatable configuration overrides through the local Docker gateway task.

Related Issue

N/A — extracted from the config-management work in #2137 following the engineering discussion.

Changes

• add repeatable dotted KEY=VALUE assignments for gateway TOML updates
• preserve comments, validate the resulting gateway schema, and replace the config atomically
• add mise run gateway:docker -- --set KEY=VALUE passthrough after generated config creation
• document the command and local development workflow
• keep multi-file config merging out of scope

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable; no sandbox infrastructure behavior changed)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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