Add SQLite skill and surface it across docs/site

[sankalpmukim]

Summary

• add a new skills/sqlite skill with practical SQLite guidance and reference docs (schema design, indexing/query planner, pragmas/tuning, WAL/checkpointing, transactions/locking, diagnostics, and migrations)
• update the root README and cursor plugin metadata to include SQLite as a first-class skill
• add SQLite to the website metadata, skill cards, and repo tree preview (including a SQLite icon)

Notes

• reviewed existing contributing guidance in README.md and followed the documented fork + PR flow

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