I wasn’t able to fix the error, but I was able to move the crash site somewhat: See the bottom for the rest of the story…

• removed the stray close-square-bracket present in 0b8736d which blocks running any Mix task
• removed all of the plt_* options from the Dialyzer config
• remove :mix and :hex from extra_applications - that one in particular silences the Could not get Core Erlang code for: /Users/mattjones/.asdf/installs/elixir/1.16.2-otp-26/.mix/archives/hex-2.1.1/hex-2.1.1/ebin/Elixir.Hex.Crypto.PublicKey.beam chatter before the main Dialyzer results
• added {:hex_core, "~> 0.11.0"}, to deps
• updated all the references to :mix_hex_* to instead use the corresponding hex_core modules named :hex_*

This results in a compiler error:

==> dialyzer_error_rep
Compiling 5 files (.ex)
     warning: Hex.Shell.info/1 is undefined (module Hex.Shell is not available or is yet to be defined)
     │
 141 │         Hex.Shell.info("* public key at #{path} does not match private key, overwriting")
     │                   ~
     │
     └─ lib/dialyzer_error_rep/registry_builder.ex:141:19: DialyzerErrorRep.RegistryBuilder.ensure_public_key/2
     └─ lib/dialyzer_error_rep/registry_builder.ex:151:17: DialyzerErrorRep.RegistryBuilder.create_directory/1
     └─ lib/dialyzer_error_rep/registry_builder.ex:158:17: DialyzerErrorRep.RegistryBuilder.write_file/2
     └─ lib/dialyzer_error_rep/registry_builder.ex:161:17: DialyzerErrorRep.RegistryBuilder.write_file/2
     └─ lib/dialyzer_error_rep/registry_builder.ex:168:15: DialyzerErrorRep.RegistryBuilder.remove_file/1

as well as the equivalent Dialyzer error:

lib/dialyzer_error_rep/registry_builder.ex:141:unknown_function
Function Hex.Shell.info/1 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:151:unknown_function
Function Hex.Shell.info/1 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:158:unknown_function
Function Hex.Shell.info/1 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:168:unknown_function
Function Hex.Shell.info/1 does not exist.
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2

Looking at the definition of Hex.Shell.info/1, it’s a trivial wrapper over Mix.shell().info/1.

Replacing those calls in RegistryBuilder causes Dialyzer to emit a new complaint:

lib/dialyzer_error_rep/registry_builder.ex:141:unknown_function
Function Mix.shell/0 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:151:unknown_function
Function Mix.shell/0 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:158:unknown_function
Function Mix.shell/0 does not exist.
________________________________________________________________________________
lib/dialyzer_error_rep/registry_builder.ex:168:unknown_function
Function Mix.shell/0 does not exist.
________________________________________________________________________________
done (warnings were emitted)

This can finally be silenced by adding plt_add_apps: [:mix], to the Dialyzer config in mix.exs, producing a clean Dialyzer report!

Generated dialyzer_error_rep app
Finding suitable PLTs
Checking PLT...
[:asn1, :compiler, :cowboy, :cowboy_telemetry, :cowlib, :crypto, :dialyzer_error_rep, :eex, :elixir, :hex_core, :inets, :kernel, :logger, :mime, :mix, :plug, :plug_cowboy, :plug_crypto, :public_key, :ranch, :ssl, :stdlib, :telemetry]
Looking up modules in dialyzer.plt
Finding applications for dialyzer.plt
Finding modules for dialyzer.plt
Checking 852 modules in dialyzer.plt
Adding 95 modules to dialyzer.plt
done in 0m3.48s
ignore_warnings: dialyzer_ignore.exs

Starting Dialyzer
[
  check_plt: false,
  init_plt: ~c"/Users/mattjones/src/dialyzer-unknown-error-rep/dialyzer_cache/dialyzer.plt",
  files: [~c"/Users/mattjones/src/dialyzer-unknown-error-rep/_build/dev/lib/dialyzer_error_rep/ebin/Elixir.DialyzerErrorRep.Application.beam",
   ~c"/Users/mattjones/src/dialyzer-unknown-error-rep/_build/dev/lib/dialyzer_error_rep/ebin/Elixir.DialyzerErrorRep.RegistryBuilder.beam",
   ~c"/Users/mattjones/src/dialyzer-unknown-error-rep/_build/dev/lib/dialyzer_error_rep/ebin/Elixir.DialyzerErrorRep.Router.beam",
   ~c"/Users/mattjones/src/dialyzer-unknown-error-rep/_build/dev/lib/dialyzer_error_rep/ebin/Elixir.DialyzerErrorRep.Static.beam",
   ~c"/Users/mattjones/src/dialyzer-unknown-error-rep/_build/dev/lib/dialyzer_error_rep/ebin/Elixir.HexRegistry.beam"],
  warnings: [:unknown]
]
Total errors: 0, Skipped: 0, Unnecessary Skips: 0
done in 0m1.33s
done (passed successfully)