## Why
The model-visible `<network>` context currently repeats indentation and
a pair of XML tags for every allowed or denied domain. Large domain sets
spend a surprising amount of prompt budget on that scaffolding instead
of the actual policy values.
## What changed
- Render allowed domains as one comma-separated `<allowed>` value
instead of one element per domain.
- Render denied domains the same way.
- Keep the full allow/deny domain sets model-visible while updating the
serialization and settings-update coverage for the denser shape.
## Example
Before:
```xml
<network enabled="true">
<allowed>api.example.test</allowed>
<allowed>cdn.example.test</allowed>
<denied>blocked.example.test</denied>
</network>
```
After:
```xml
<network enabled="true"><allowed>api.example.test,cdn.example.test</allowed><denied>blocked.example.test</denied></network>
```
## Validation
- `cargo test -p codex-core environment_context`
- `cargo test -p codex-core
build_settings_update_items_emits_environment_item_for_network_changes`
- Ran a local `codex` session with a real network context containing 121
allowed domains and 42 denied domains, then inspected the raw prompt
with `raw_token_viewer_cli.py`. With the same domain set, the rendered
`<network>` section shrank from 7,175 characters across 161 lines to
3,666 characters on one line, and the containing environment-context
block fell from 6,428 tokens to 5,379 tokens.
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