Fix docs and config out of sync with new features

[shahcolate]

1. Add pm-interview-prep to .claude-plugin/marketplace.json
2. Add [1.4.0] entry to CHANGELOG.md with all new features
3. Add --baseline docs and update case count in evals/README.md
4. Reference pm-interview-prep in CONTRIBUTING.md plugin examples
5. Add --baseline checkbox to eval.yml workflow_dispatch inputs
6. Remove non-existent releases/ dir from README repo structure

Pull Request

Summary

What does this PR change, and why?

Type of change

• Improving an existing plugin (new framework, edge case, anti-pattern, fix)
• New plugin
• Documentation fix
• Infrastructure / tooling

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Checklist

All PRs

• CHANGELOG.md updated under ## [Unreleased]
• No broken links in any modified .md files

Plugin changes (new or updated)

• plugin.json updated (version bumped if this is a content change)
• SKILL.md instructions are in imperative form ("Do X", not "You should X")
• Each new framework includes: when to apply it, what it produces, an example
• Anti-patterns covered for any new framework
• Confidence labels (🟢 High / 🟡 Medium / 🔴 Low) used where appropriate

New plugins only

• .claude-plugin/plugin.json created with correct name, version, description, skills
• SKILL.md file exists at the path referenced in plugin.json
• Example conversations included (before/after the skill is installed)
• Plugin listed in root marketplace.json

Test conversations

Paste 1–2 example prompts and Claude's response with this skill active. This is the quality bar — if you can't show it working well, it's not ready.

Prompt 1:

[your test prompt]

Response:

[Claude's actual output]

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Breaking changes

Does this change alter existing behavior? If yes, describe what changes and why it's worth the disruption.