Fix linting issues: canonicalheader, embeddedstructfieldcheck, iotamixing, makezero, testableexamples, wastedassign, usetesting, tparallel, unconvert, intrange, iface

- canonicalheader: fix cacheTTL header value to canonical form
- embeddedstructfieldcheck: move embedded fields before regular fields in 4 structs
- iotamixing: split const blocks mixing iota with non-iota constants
- makezero: use make([]T, 0, n) instead of make([]T, n) before appending
- testableexamples: add missing Output: comment to ExampleOption_UnwrapOrZero
- wastedassign: remove wasted initial assignments in 4 locations
- usetesting: replace os.MkdirTemp/os.Setenv with t.TempDir/t.Setenv in tests
- tparallel: add t.Parallel() to 8 top-level test functions
- unconvert: remove 16 unnecessary type conversions
- intrange: convert 3 for loops to use integer range syntax
- iface: consolidate identical EditPrompter and Prompt interfaces via type alias

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

pkg/cmd/ssh-key/shared/user_keys.go

@@ -37,7 +37,7 @@ func UserKeys(httpClient *http.Client, host, userHandle string) ([]sshKey, error
 		return nil, err
 	}
 
-	for i := 0; i < len(keys); i++ {
+	for i := range keys {
 		keys[i].Type = AuthenticationKey
 	}
 
@@ -57,7 +57,7 @@ func UserSigningKeys(httpClient *http.Client, host, userHandle string) ([]sshKey
 		return nil, err
 	}
 
-	for i := 0; i < len(keys); i++ {
+	for i := range keys {
 		keys[i].Type = SigningKey
 	}