docs: register the MCP server from an installed release

[sawizzle]

The MCP docs only show registering pdfcraft-cli from a source build (target/release/...). People who
install a release (MSI, deb/rpm, the macOS CLI zip) have to work out where the CLI lives. On Windows
the MSI doesn't add it to PATH, and on macOS the CLI isn't inside the .app.

This adds a short "From an installed release" section to crates/automation/README.md, covering:

• where each package installs the CLI by default (taken from packaging/windows/*.wxs,
  packaging/linux/nfpm.yaml and packaging/macos/package.sh);
• the matching claude mcp add commands.

It's a docs-only change, +17 lines, nothing removed. I tested the Windows path and command on Windows 11
with the 2026-10-08 release MSI. The Linux and macOS paths come from the packaging files; I haven't run them.

🤖 Generated with Claude Code

[echelon]

Thank you, @sawizzle! This has landed on main: "docs: register the MCP server from an installed release".

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