Add custom writing instructions for report generation

[SiriusYou]

Summary

• Adds a Custom Writing Instructions textarea to the frontend UI, allowing users to specify style, tone, and formatting directives for generated reports
• Instructions are threaded through the full stack (frontend → WebSocket → server → report handlers → writer → prompt injection) and only affect report writing, not research/retrieval
• Supports Summary and Detailed report types with a guard that skips unsupported types gracefully

Changes

File	Change
frontend/index.html	Added textarea after Query Domains input
frontend/scripts.js	Sends custom_instructions field in WebSocket request
backend/server/server_utils.py	Extracts and passes custom_instructions from request data
backend/server/websocket_manager.py	Threads custom_instructions through to report handlers with type guard
backend/report_type/basic_report/basic_report.py	Accepts and forwards custom_instructions to write_report()
backend/report_type/detailed_report/detailed_report.py	Accepts and forwards to intro, subtopic, and conclusion writers
gpt_researcher/agent.py	Adds custom_instructions parameter to 3 write methods
gpt_researcher/skills/writer.py	Forwards custom_instructions to action functions
gpt_researcher/actions/report_generation.py	Core prompt injection — appends instructions to system prompt

Test plan

• Start backend with python -m uvicorn main:app --reload
• Open frontend, enter a query with custom instructions (e.g., "Write in bullet points, use formal tone")
• Run a Summary report — verify instructions appear in generated output style
• Run a Detailed report — verify instructions affect intro, subtopics, and conclusion
• Run a report type other than Summary/Detailed — verify it works normally without error
• Leave custom instructions empty — verify default behavior unchanged

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