[codex] remove dead local integration code

[aibrahim-oai]

Summary

• remove the unused local in-process RMCP transport and its unconstructed factory/recipe variants
• remove the redundant EffectiveSkillRoots API and tests that only exercised that dead path
• delete a test-only apps renderer with no production callers
• narrow one unused skills string export and remove a stale TUI dead-code suppression
• explicitly preserve all remote/network-facing functionality and every line introduced by jif <jif@openai.com>

Scope guardrails

This revision restores all remote skills, remote plugin mutation/sync, remote marketplace, Streamable HTTP/OAuth, custom MCP request/notification, Codex Apps auth, and connector snapshot functionality.

A deleted-line provenance audit against the PR base found no remaining deletion authored by Jif. Jif's connector snapshot and tests from #29851 are unchanged.

Validation

• deleted-line git blame audit: zero remaining Jif-authored deletions
• repository-wide symbol audit confirms remote and Jif-owned APIs remain present
• just test -p codex-rmcp-client -p codex-plugin -p codex-core-plugins: 388 passed; one harness prerequisite failure
• built codex-cli, then reran the exact remote round-trip test: passed
• all-target checks for RMCP, plugin/skills, core, app-server, and TUI: passed
• scoped just fix: passed
• just fmt: passed
• git diff --check: passed

Final code delta: 3 insertions, 160 deletions across 13 files.