add experimental path to skill installer

[JustinSmethers]

Update skill-installer guidance to cover experimental skills listing

This PR updates codex-rs/core/src/skills/assets/samples/skill-installer/SKILL.md to document how to list skills from skills/.experimental using list-curated-skills.py --path skills/.experimental or list-curated-skills.py experimental (output documented below).

This addresses the issue where the plan skill moved to .experimental in 0.77 and is no longer discoverable via the skill-installer workflow.

Issue: Link

Validation output (local rebuild)

$ skill-installer experimental

[approved] You approved codex to run /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-cur... this time

- Ran /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py --path
  | skills/.experimental
  ` zsh:1: permission denied: /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py

[approved] You approved codex to run python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py --path skills/.experimental this time

- Ran python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py --path
  | skills/.experimental
  ` 1. create-plan
    2. linear

--- Worked for 14s ---

- Skills from https://github.com/openai/skills/tree/main/skills/.experimental:

  1. create-plan
  2. linear

  Which ones would you like installed?

$ skill-installer --path=skills/.experimental

- You provided the experimental channel path but not which skill(s) to install. Which of these should I install: create-plan or linear?

$ skill-installer create-plan

[approved] You approved codex to run python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/install-skill-from-github.py this time

- Ran python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/install-skill-from-github.py
  | --repo openai/skills --path skills/.experimental/create-plan
  ` Installed create-plan to /Users/justin.smethers/.codex/skills/create-plan

--- Worked for 7s ---

- Installed create-plan to /Users/justin.smethers/.codex/skills/create-plan. Restart Codex to pick up new skills.

$ skill-installer

[approved] You approved codex to run python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py this time

- Ran python3 /Users/justin.smethers/.codex/skills/.system/skill-installer/scripts/list-curated-skills.py
  ` 1. gh-address-comments
    2. gh-fix-ci
    ... +2 lines
    5. notion-research-documentation
    6. notion-spec-to-implementation

--- Worked for 7s ---

- Skills from https://github.com/openai/skills/tree/main/skills/.curated:

  1. gh-address-comments
  2. gh-fix-ci
  3. notion-knowledge-capture
  4. notion-meeting-intelligence
  5. notion-research-documentation
  6. notion-spec-to-implementation

  Which ones would you like installed?

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[JustinSmethers]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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