feat(pi): register codex MCP server so codex-review works from Pi

[jinyeow]

What

codex-review (moved to the portable layout in #98) now projects to ~/.pi/agent/skills/codex-review/, but its precondition (a working codex MCP server) is only registered for Claude Code (claude mcp add, in both installers). Pi has no equivalent MCP registration in pi/settings.json/pi/extensions/, so the skill is inert there today — Pi can list it but invoking it fails the precondition.

Note: this is Pi asking Codex for a second opinion, same as Claude Code does — not Codex reviewing itself. Codex CLI itself is correctly excluded from getting this skill at all (self-review doesn't make sense), per #98's installer change.

Why

#98's quick-review flagged this as a MEDIUM (conventions) finding: the gap is disclosed in three places (ai-agents/SKILL-OWNERSHIP.md, codex/README.md, ai-agents/skills/codex-review/SKILL.md) but had no tracking ticket, unlike every other open gap in SKILL-OWNERSHIP.md (#94, #97, #99, #100 are all referenced there).

Scope

• Register the codex MCP server for Pi (check Pi's MCP config mechanism — likely pi/settings.json or an extension, mirroring how Claude Code's claude mcp add is installed by setup.ps1/setup.sh).
• Confirm codex-review's precondition check actually passes on Pi once registered.
• Update the three docs that currently disclose this gap (ai-agents/SKILL-OWNERSHIP.md, codex/README.md, ai-agents/skills/codex-review/SKILL.md) to reference this ticket instead of loose prose, and remove the caveat once fixed.

Acceptance criteria

• Pi has a working codex MCP registration installed by setup.ps1/setup.sh.
• codex-review actually runs successfully from a Pi session (precondition passes).
• The three docs above no longer describe this as an open gap.