refactor(context): modularize context gathering and add search/replace output to quick_chat

- Extracts core context logic into separate modules: `base.lua`, `json_formatter.lua`, `plain_text_formatter.lua`
- Updates `quick_chat` to output and apply LLM-generated SEARCH/REPLACE blocks (instead of JSON object replacement)
- Improves context formatting for both LLM-friendly plain text and API JSON
- Refactors quick chat flow for simpler, more robust edit workflows

Author: sudo-tee