fix: make proxy config writing idempotent

[fallintoplace]

Summary

• Replace action-managed proxy config sections instead of adding duplicates on repeated runs.
• Migrate the previous emitted marker format to the new managed sections.
• Preserve unrelated settings and propagate non-missing-file read errors.

Why

Repeated config writes could duplicate the root provider selection and proxy provider table when the same home directory was reused. The new writer owns only the action-managed sections and leaves unrelated settings intact.

Tests

• npx pnpm@10.33.0 run check
• npx pnpm@10.33.0 test

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[fallintoplace]

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[fallintoplace]

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