Link skills to experience entries

[yawboakye]

Summary

• add a developers_experience_skills join model and table with a unique experience/profile-skill pair and cascading foreign keys
• enforce that linked profile skills belong to the same developer profile as the experience
• add a secure multi-select limited to the current profile's skills when creating or editing experience
• show linked skills as chips beneath each experience entry on the public profile, with links to the matching developer-directory filter

Screenshots

Desktop

Mobile

Issue requirements

• Developers can link and unlink their own profile skills on experience entries
• Skills from another profile are rejected by model validation and secure association handling
• Public profiles show linked skills under each experience
• Removing either the profile skill or experience removes the join records
• The join table declares its unique index and foreign keys inside create_table

Testing

• Focused model and integration tests: 20 runs, 131 assertions, 0 failures
• Non-image Rails suite: 229 runs, 1,204 assertions, 0 failures
• bundle exec standardrb
• yarn build
• yarn build:css

The complete local Rails run reached 235 tests with no assertion failures, but four OG-image tests errored because this machine does not have the system libvips library. Those image paths are unrelated to this change.

Closes #446