Show ucm, not unison, as the program name in help and messages

.github/workflows/proofs/formatting/132c5c459fe96db10bf3734c42e5b7c22da499f21947b0ecabc942cf66d06e27.txt

@@ -0,0 +1 @@
+ab5370655049c955 pass 132c5c459fe96db10bf3734c42e5b7c22da499f21947b0ecabc942cf66d06e27

.github/workflows/proofs/tests/5d5fcc4f2dc239f0b0545d951f2892cd5486e42e0db3ef31842edfdf5e765f65.txt

@@ -0,0 +1 @@
+d4d2c585d96c2f39 pass 5d5fcc4f2dc239f0b0545d951f2892cd5486e42e0db3ef31842edfdf5e765f65

.github/workflows/proofs/transcripts/864d04ecf65a3919b39c5507438050e8f622bd4f33bd0f16986860e579844e7c.txt

@@ -0,0 +1 @@
+fe1fa86f2d4650c1 pass 864d04ecf65a3919b39c5507438050e8f622bd4f33bd0f16986860e579844e7c

unison-cli/src/ArgParse.hs

@@ -52,7 +52,7 @@ import Options.Applicative.Builder.Internal (noGlobal {- https://github.com/pcap
 import Options.Applicative.Help (bold, (<+>))
 import Options.Applicative.Help.Pretty qualified as P
 import Stats
-import System.Environment (lookupEnv)
+import System.Environment (lookupEnv, withProgName)
 import Text.Megaparsec qualified as Megaparsec
 import Unison.Codebase.Path qualified as Path
 import Unison.Codebase.Path.Parse qualified as Path
@@ -164,7 +164,8 @@ parseCLIArgs progName version = do
   let parserInfo = rootParserInfo progName version envOpts
   let preferences = prefs $ showHelpOnError <> helpShowGlobals <> columns cols <> subparserInline
   let usage = renderUsage progName parserInfo preferences
-  (globalOptions, command) <- customExecParser preferences parserInfo
+  -- optparse-applicative looks the program name up itself, for the usage lines it prints
+  (globalOptions, command) <- withProgName progName $ customExecParser preferences parserInfo
   pure $ (usage, globalOptions, command)
 
 -- | Load default options from environment variables.

unison-cli/src/Unison/Main.hs

@@ -164,7 +164,7 @@ main version = do
     interruptHandler <- defaultInterruptHandler
     withInterruptHandler interruptHandler $ do
       void $ Ki.fork scope (initHTTPClient version)
-      progName <- getProgName
+      progName <- getUserFacingProgName
       -- hSetBuffering stdout NoBuffering -- cool
       (renderUsageInfo, globalOptions, command) <- parseCLIArgs progName (Text.unpack (Version.gitDescribeWithDate version))
       let GlobalOptions {codebasePathOption = mCodePathOption, exitOption, lspFormattingConfig, jitOption, jitDumpIROption} = globalOptions
@@ -457,6 +457,16 @@ initHTTPClient version = do
   manager <- HTTP.newTlsManagerWith managerSettings
   HTTP.setGlobalManager manager
 
+-- | The name to show users for this program, in help text and messages.
+--
+-- The release bundle and the Nix package start the @unison@ executable from a @ucm@ launcher, so 'getProgName' gives
+-- @unison@, a command users don't have.
+getUserFacingProgName :: IO String
+getUserFacingProgName =
+  getProgName <&> \case
+    "unison" -> "ucm"
+    name -> name
+
 -- | Prep the codebase for transcripts, then pass the directory to the action.
 -- After the action the codebase will be deleted/copied/saved as indicated.
 withTranscriptDir :: Verbosity.Verbosity -> String -> TranscriptCodebaseSetup -> Maybe CodebasePathOption -> (FilePath -> IO r) -> IO (Maybe r)
@@ -642,7 +652,7 @@ runTranscripts jit version verbosity renderUsageInfo codebaseSetup mCodePathOpti
       putStrLn (renderUsageInfo $ Just "transcript")
       Exit.exitWith (Exit.ExitFailure 1)
     Success markdownFiles -> pure markdownFiles
-  progName <- getProgName
+  progName <- getUserFacingProgName
   completed <-
     fromMaybe False <$> withTranscriptDir verbosity progName codebaseSetup mCodePathOption \transcriptDir -> do
       runTranscripts' jit version progName transcriptDir markdownFiles
@@ -714,7 +724,7 @@ getCodebaseOrExit codebasePathOption locking migrationStrategy action = do
     Left (dir, err) ->
       let message = do
             pDir <- prettyDir dir
-            executableName <- P.text . Text.pack <$> getProgName
+            executableName <- P.text . Text.pack <$> getUserFacingProgName
 
             case err of
               InitErrorOpen err ->