Add Codex-native onboarding docs and replace Claude Code install path

[ethanolivertroy]

Context

Current onboarding is Claude-specific:

• README.md says this is a Claude Code plugin marketplace.
• docs/QUICKSTART.md requires Claude Code and /plugin marketplace add GRCEngClub/claude-grc-engineering.
• Docs point to docs/CLAUDE-COWORK.md for non-Claude Code usage.

Codex app users need a different path. They typically operate from repository context, task prompts, terminal commands, and repo instructions—not Claude Code /plugin marketplace installation.

Tasks

• Add a Codex-first quickstart, e.g. docs/CODEX-QUICKSTART.md.
• Update README.md top section for codex-grc-engineering identity.
• Replace or clearly mark Claude Code /plugin commands as legacy/source-fork instructions.
• Add example Codex task prompts for common GRC workflows:
  • gap assessment
  • IaC scan
  • evidence collection
  • control mapping
  • OSCAL/FedRAMP output generation
• Update links from GRCEngClub/claude-grc-engineering to GRCEngClub/codex-grc-engineering where appropriate.
• Add a warning that not every Claude harness workflow is expected to work identically in Codex.

Acceptance Criteria

• A new user can open the repo in Codex and understand how to run the first supported workflow without Claude Code plugin commands.
• README and Quickstart no longer present Claude Code as the only primary path.