## Summary
Implements Linux bubblewrap support for restricted `ReadOnlyAccess`
(introduced in #11387) by honoring `readable_roots` and
`include_platform_defaults` instead of failing closed.
## What changed
- Added a Linux platform-default read allowlist for common
system/runtime paths (e.g. /usr, /etc, /lib*, Nix store roots).
- Updated the bwrap filesystem mount builder to support restricted read
access:
- Full-read policies still use `--ro-bind / /`
- Restricted-read policies now start from` --tmpfs `/ and add scoped
`--ro-bind` mounts
- Preserved existing writable-root and protected-subpath behavior
(`.git`, `.codex`, etc.).
`ReadOnlyAccess::Restricted` was already modeled in protocol, but Linux
bwrap still returned `UnsupportedOperation` for restricted read access.
This closes that gap for the active Linux filesystem backend.
## Notes
Legacy Linux Landlock fallback still fail-closes for restricted read
access (unchanged).
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