Remove invalid system-fragment flag from plugin commands and templates

[julwrites]

• Removed M.get_system_fragment_args from lua/llm/commands.lua
• Removed --system-fragment parameter processing in lua/llm/managers/templates_manager.lua
• Removed system fragment UI selections from lua/llm/ui/views/templates_view.lua
• Updated test specs in tests/spec/managers/templates_manager_spec.lua and tests/spec/ui/views/templates_view_spec.lua
• Cleaned up completed item "Tech Debt 4" from BACKLOG.md and reordered remaining ranked items.

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PR created automatically by Jules for task 17231252163810454296 started by @julwrites

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