feat: replace Rust substrate with Codex base

[shiny-code-bot]

Summary

• replace code-rs with the current Codex-base Rust workspace while keeping codex-rs as the read-only upstream mirror
• keep imported upstream crates internally named codex-*, but build the Every Code CLI binary as code
• preserve the foundation Every Code home policy: CODE_HOME > CODEX_HOME > ~/.code
• narrow the codex path dependency guard so it blocks actual sibling ../codex-rs references without rejecting local imported codex-* crates

Scope notes

• This is the Plan Codex-base substrate migration for Every Code #384 substrate foundation slice, not the Every Code feature-port slice.
• Inside code-rs, the only content differences from the local codex-rs mirror are cli/Cargo.toml, cli/src/main.rs, and utils/home-dir/src/lib.rs.
• User-facing product copy/version cleanup and Every Code feature ports are intentionally left for follow-up slices.

Validation

• ./build-fast.sh
• cargo test -p codex-utils-home-dir --locked
• Review agent: substrate foundation review, not merge-blocked
• Review agent: final rereview, not merge-blocked