start shell-integration folder, copy into zig-out

src/shell-integration/README.md

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+# Shell Integration Code
+
+This is the shell-specific shell-integration code that is
+used for the shell-integration feature set that Ghostty
+supports.
+
+This README is meant as developer documentation and not as
+user documentation. For user documentation, see the main
+README.
+
+## Implementation Details
+
+### Fish
+
+For [Fish](https://fishshell.com/), Ghostty prepends to the
+`XDG_DATA_DIRS` directory. Fish automatically loads configuration
+files in `<XDG_DATA_DIR>/fish/vendor_conf.d/*.fish` on startup,
+allowing us to automatically integrate with the shell. For details
+on the Fish startup process, see the
+[Fish documentation](https://fishshell.com/docs/current/language.html).

src/shell-integration/fish/vendor_conf.d/ghostty-shell-integration.fish

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+#!/bin/fish
+
+echo GHOSTTY INTEGRATION LOADING