feat(codex): add Agent Plugins example - #1898

• Update the pinned Codex build and expose existing agentOS POSIX behavior through Rust std, libc, networking, file-lock, and PTY paths.
• Add an end-to-end portable Agent Plugin example, regression test, and sessions documentation.
• Replace stock-WASI capability fallbacks with real agentOS behavior and document the rule for future patches.

Validation:

• Agent Plugin Codex full-turn test
• agentOS kernel advisory-lock and PTY resize tests
• website build and fixed-version verification