fix(notfiles): save state on partial link failure; skip re-copy of unchanged files

Closes #8, closes #9

scripts/preflight.nu

@@ -0,0 +1,37 @@
+#!/usr/bin/env nu
+# preflight.nu — notfiles environment validation
+
+def check [label: string, pass: bool, detail: string = ""] {
+    if $pass {
+        print $"[ok]   ($label)"
+    } else if $detail != "" {
+        print $"[fail] ($label) — ($detail)"
+    } else {
+        print $"[fail] ($label)"
+    }
+    $pass
+}
+
+print "=== notfiles preflight ==="
+
+let results = [
+    (check "cargo on PATH" (which cargo | length) > 0),
+    (check "just on PATH" (which just | length) > 0),
+    (check "nu on PATH" (which nu | length) > 0 "nushell required for hook scripts"),
+    (check "age on PATH" (which age | length) > 0 "optional — needed for SOPS decryption"),
+    (check "sops on PATH" (which sops | length) > 0 "optional — needed for encrypted configs"),
+    (check "notfiles on PATH" (which notfiles | length) > 0 "run: cargo install --path crates/notfiles"),
+    (check "op on PATH" (which op | length) > 0),
+    (check "1Password authed" (do { op account list } | complete | get exit_code) == 0),
+    (check "git repo clean" (do { git status --porcelain } | complete | get stdout | str trim | is-empty)),
+]
+
+let failed = $results | where { |r| not $r } | length
+let total = $results | length
+
+print ""
+if $failed == 0 {
+    print $"preflight passed ($total)/($total)"
+} else {
+    print $"preflight ($total - $failed)/($total) — ($failed) check(s) failed"
+}