Agent Host: Support custom Codex permissions from config.toml

When using Codex through Agent Host, the permissions selector currently offers only Default Permissions, Auto-Review, and Full Access. Each preset expands to explicit approval, sandbox, and reviewer values, so users cannot choose the permissions they already maintain in Codex config.toml.

Agent Host should offer a Custom permissions option that uses Codex's effective configuration:

- New threads should avoid overriding the configured permission values.
- Subsequent turns, resumed sessions, and forks should use the current effective permission configuration instead of retaining stale thread values.
- Both legacy approval_policy / sandbox_mode configuration and named default_permissions profiles should be supported.
- Existing Default Permissions, Auto-Review, and Full Access behavior should remain unchanged.

Only the effective permission projection should be passed to thread operations. Other configuration, including credentials and MCP server definitions, should continue to be owned and loaded by Codex.

This would let Agent Host users share the same permission policy they use with the Codex CLI and app without duplicating it in every VS Code session.