docs: replace install-mcp with aix add mcp

[yokuze]

This change replaces all instances of the install-mcp command with aix add mcp.
aix is a more robust tool for managing AI assistant configurations across multiple editors.

Changes:

• Replaced [install-mcp](https://www.npmjs.com/package/install-mcp) with [aix](https://aix.a1st.dev/cli/add/#aix-add-mcp)
• Replaced npx -y install-mcp @hypothesi/tauri-mcp-server --client <client> with npx -y @a1st/aix add mcp @hypothesi/tauri-mcp-server --user
• Consolidated redundant client-specific installation blocks in the root README.md into a single simplified section.
• Updated documentation in:
  • README.md
  • docs/index.md
  • docs/guides/getting-started.md
  • packages/mcp-server/README.md

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PR created automatically by Jules for task 1592587288481737353 started by @yokuze

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[yokuze]

Closes #26