Ignore proc-macro-error2 advisory (#26974)

## Summary
- ignore RUSTSEC-2026-0173 in cargo-deny and cargo-audit config
- document that proc-macro-error2 is pulled in transitively via
i18n-embed-fl/age/codex-secrets
- leave the ignore temporary until codex-secrets moves off age or age
drops i18n-embed-fl

## Validation
- just fmt
- cargo deny check --hide-inclusion-graph

codex-rs/.cargo/audit.toml

@@ -9,4 +9,5 @@ ignore = [
     "RUSTSEC-2025-0141", # bincode via syntect; remove when syntect drops or updates it
     "RUSTSEC-2026-0118", # hickory-proto via rama-dns/rama-tcp; remove when rama updates to hickory 0.26.1 or hickory-net
     "RUSTSEC-2026-0119", # hickory-proto via rama-dns/rama-tcp; remove when rama updates to hickory 0.26.1 or hickory-net
+    "RUSTSEC-2026-0173", # proc-macro-error2 via i18n-embed-fl/age/codex-secrets; remove when local secrets storage migrates off age or age drops i18n-embed-fl
 ]

codex-rs/deny.toml

@@ -81,6 +81,7 @@ ignore = [
     { id = "RUSTSEC-2025-0141", reason = "bincode is unmaintained; pulled in via syntect v5.3.0 used by codex-tui for syntax highlighting; no fixed release yet" },
     { id = "RUSTSEC-2026-0118", reason = "hickory-proto v0.25.2 is pulled in via rama-dns/rama-tcp used by codex-network-proxy; DNSSEC features are not enabled; remove when rama updates to hickory 0.26.1 or hickory-net" },
     { id = "RUSTSEC-2026-0119", reason = "hickory-proto v0.25.2 is pulled in via rama-dns/rama-tcp used by codex-network-proxy; no fixed rama release is available yet; remove when rama updates to hickory 0.26.1 or hickory-net" },
+    { id = "RUSTSEC-2026-0173", reason = "proc-macro-error2 is unmaintained; pulled in via i18n-embed-fl/age used by codex-secrets local storage; remove when codex-secrets migrates off age or age drops i18n-embed-fl" },
 ]
 # If this is true, then cargo deny will use the git executable to fetch advisory database.
 # If this is false, then it uses a built-in git library.