feat: add conflict resolution, linter validation, and SEARCH/REPLACE system

- Add git-style conflict resolution with visual diff highlighting
- Add buffer-local keymaps: co/ct/cb/cn for conflict resolution
- Add floating menu with auto-show after code injection
- Add linter validation that auto-checks LSP diagnostics after accepting code
- Add SEARCH/REPLACE block parsing with fuzzy matching
- Add new commands: CoderConflictMenu, CoderLintCheck, CoderLintFix
- Update README with complete keymaps reference and issue reporting guide
- Update CHANGELOG and llms.txt with full documentation
- Clean up code comments and documentation

Co-Authored-By: Claude <noreply@anthropic.com>

lua/codetyper/config.lua

@@ -20,7 +20,7 @@ local defaults = {
       model = "gemini-2.0-flash",
     },
     copilot = {
-      model = "gpt-4o", -- Uses GitHub Copilot authentication
+      model = "claude-sonnet-4", -- Uses GitHub Copilot authentication
     },
   },
   window = {