Remove stale 8912 listener references

[yvette-carlisle]

Summary

• change the CLI default serve listener to 127.0.0.1:8192
• replace stale 8912 examples in README, runbooks, and Decodex plugin skills
• add a CLI parsing regression test for the default serve listener

Validation

• cargo test -p decodex parses_serve_default_listen_address
• cargo run -p decodex --bin decodex -- serve --help | rg -n "listen|8192|8912"
• rg -n "8912" . /Users/x/.codex/plugins/cache/hack-ink/decodex/0.1.0 returned no matches
• python3 /Users/x/.codex/plugins/cache/hack-ink/playbook/0.3.0/scripts/semantic_drift_audit.py --rev HEAD^
• cargo make check