Establish project foundation: domain model, MVI spec, environment reference

Capture the design work for the NixOS migration before any implementation:

- .claude/CONTEXT.md: domain glossary (Host, Module, Skeleton, Auto-loader,
  Enable convention, unstable/stable overlay)
- .claude/adr/0001-sops-nix-for-secrets.md: secrets tooling decision
- .claude/spec/laptop-mvi.md: frozen minimum-viable-install spec for neogaia

- reference/: read-only snapshot of the current CachyOS configs (secrets and
  state excluded), plus ENVIRONMENT.md profiling the live environment to guide
  replication

reference/home/.config/fish/functions/fishtape.fish

@@ -0,0 +1,116 @@
+function fishtape --description "Test scripts, functions, and plugins in Fish"
+    switch "$argv"
+        case -v --version
+            echo "fishtape, version 3.0.1"
+        case "" -h --help
+            echo "Usage: fishtape <files ...>  Run test files"
+            echo "Options:"
+            echo "       -v or --version  Print version"
+            echo "       -h or --help     Print this help message"
+        case \*
+            set --local files (realpath $argv)
+
+            for file in $files
+                if test ! -f $file
+                    echo "fishtape: Invalid file or file not found: \"$file\"" >&2
+                    return 1
+                end
+            end
+
+            set --local operators -{n,z,b,c,d,e,f,g,G,k,L,O,p,r,s,S,t,u,w,x}
+            set --local expectations \
+                "a non-zero length string" \
+                "a zero length string" \
+                "a block device" \
+                "a character device" \
+                "a directory" \
+                "an existing file" \
+                "a regular file" \
+                "a file with the set-group-ID bit set" \
+                "a file with same group ID as the current user" \
+                "a file with the sticky bit set" \
+                "a symbolic link" \
+                "a file owned by the current user" \
+                "a named pipe" \
+                "a file marked as readable" \
+                "a file of size greater than zero" \
+                "a socket" \
+                "a terminal tty file descriptor" \
+                "a file with the set-user-ID bit set" \
+                "a file marked as writable" \
+                "a file marked as executable"
+
+            set --universal _fishtape_test_number 0
+            set --universal _fishtape_test_passed 0
+            set --universal _fishtape_test_failed 0
+
+            function @echo
+                echo "# $argv"
+            end
+
+            function @test --argument-names name --inherit-variable operators --inherit-variable expectations
+                set --erase argv[1]
+                set --query argv[2] || set --append argv ""
+
+                set _fishtape_test_number (math $_fishtape_test_number + 1)
+
+                if test $argv
+                    set _fishtape_test_passed (math $_fishtape_test_passed + 1)
+
+                    echo "ok $_fishtape_test_number $name"
+                else
+                    if test $argv[1] = "!"
+                        set operator "! "
+                        set expected "not "
+                        set --erase argv[1]
+                    end
+
+                    if set --query argv[3]
+                        set operator "$operator"$argv[2]
+                        set expected (string escape -- $argv[3])
+                        set actual (string escape -- $argv[1])
+                    else
+                        set operator "$operator"$argv[1]
+                        set expected "$expected"$expectations[(contains --index -- $argv[1] $operators)]
+                        set actual (string escape -- $argv[2])
+                    end
+
+                    set _fishtape_test_failed (math $_fishtape_test_failed + 1)
+
+                    status print-stack-trace |
+                        string replace --filter --regex -- "\s+called on line (\d+) of file (.+)" '$2:$1' |
+                        read --local at
+
+                    echo "not ok $_fishtape_test_number $name"
+                    echo "  ---"
+                    echo "    operator: $operator"
+                    echo "    expected: $expected"
+                    echo "    actual: $actual"
+                    echo "    at: $at"
+                    echo "  ..."
+                end
+            end
+
+            echo TAP version 13
+
+            for file in $files
+                fish --init-command=(functions @echo | string collect) --init-command=(functions @test | string collect) $file
+            end
+
+            echo
+            echo "1..$_fishtape_test_number"
+            echo "# pass $_fishtape_test_passed"
+            test $_fishtape_test_failed -eq 0 &&
+                echo "# ok" ||
+                echo "# fail $_fishtape_test_failed"
+
+            functions --erase @echo @test
+
+            set --local failed $_fishtape_test_failed
+            set --erase _fishtape_test_number
+            set --erase _fishtape_test_passed
+            set --erase _fishtape_test_failed
+
+            test $failed -eq 0
+    end
+end