feat(convert): document RTK token-saving alternatives per target tool

[duyluann]

Problem

rtk is a Claude Code-specific CLI proxy that filters command output to reduce token usage (60-90% savings). No equivalent exists in Gemini CLI, GitHub Copilot, or OpenAI Codex.

Currently /convert marks this as unsupported with no guidance.

Proposed Solution

Document tool-specific context management strategies in each generated config file:

• Gemini CLI: model.summarizeToolOutput in .gemini/settings.json can cap token output per tool (e.g., limit run_shell_command output to N tokens)
• Copilot: Instruct users to use # file references instead of pasting full file contents
• Codex: Document context window best practices for large repos

Acceptance Criteria

• Each generated target config includes a "Token Management" section
• Gemini CLI section includes summarizeToolOutput config example
• Copilot/Codex sections include practical tips

Ref: #11