refactor: fix reliability bugs and reduce code duplication

[CoderCookE]

• Fix VimScript injection: display content now passed via vim.vars instead of string-formatted into vim.command, preventing crashes on backslashes
• Fix broken :GptBe persona command: was setting g:chat_persona which Python never read; now correctly sets g:llm_agent_persona
• Fix Anthropic URL construction: use urlparse to handle bare domains, /v1 paths, and full /v1/messages URLs without double-appending
• Fix save_plan() bypassing get_config(): now respects g:llm_agent_session_mode
• Fix window focus hijack: display_response() now restores original window after scrolling the chat buffer
• Fix path prefix guard in context.vim: replace length comparison with stridx() to avoid incorrectly skipping short paths like /srv or /app
• Remove ~20 lines of unreachable dead code after unconditional break
• Extract append_tool_results() shared helper to utils.py, eliminating duplicate tool-loop logic and dead format-detection branch in context.py
• Centralize Python sys.path setup into chatgpt#ensure_python_path(), removing 8 copy-pasted heredoc blocks across autoload files
• Fix conftest history fixture to use real \x01 delimiters
• Relax tool count assertion to >= 17
• Correct README default for summary_compaction_size (50KB -> 75KB)