Add Codex tool support to convert/install workflow

[gorlev]

What does this PR do?

Adds first-class Codex integration to the existing convert/install workflow without changing the current installation model.

Included changes

• Add --tool codex to scripts/convert.sh and generate Codex skills at integrations/codex/agency-<slug>/SKILL.md
• Add --tool codex to scripts/install.sh and install to ~/.codex/skills/
• Add Codex detection and installer menu label
• Add guardrails for missing generated Codex skills
• Update docs (README.md, integrations/README.md) to include Codex in the same flow as other tools
• Add integrations/codex/README.md
• Update .gitignore to ignore generated Codex artifacts while keeping docs tracked

Validation

• bash -n scripts/convert.sh
• bash -n scripts/install.sh
• ./scripts/convert.sh --tool codex
• ./scripts/install.sh --tool codex --no-interactive

Agent Information (if adding/modifying an agent)

• Agent Name: N/A (no agent added/modified)
• Category: N/A
• Specialty: N/A

Checklist

• Follows the agent template structure from CONTRIBUTING.md (N/A, no agent file change)
• Includes YAML frontmatter with name, description, color (N/A, no agent file change)
• Has concrete code/template examples (for new agents) (N/A)
• Tested in real scenarios (Applied to integration scripts/docs workflow)
• Proofread and formatted correctly