refactor: move CLI code from cmd/ to internal/cli

Move all non-bootstrapping CLI code to internal/cli package.
cmd/neoirc-cli/main.go now contains only minimal bootstrapping
that calls cli.Run(). The App struct, UI, command handlers, poll
loop, and api client are now in internal/cli/ and internal/cli/api/.

README.md

@@ -2311,15 +2311,18 @@ neoirc/
 ├── cmd/
 │   ├── neoircd/            # Server binary entry point
 │   │   └── main.go
-│   └── neoirc-cli/         # TUI client
-│       ├── main.go         # Command handling, poll loop
-│       ├── ui.go           # tview-based terminal UI
-│       └── api/
-│           ├── client.go   # HTTP API client library
-│           └── types.go    # Request/response types
+│   └── neoirc-cli/         # TUI client entry point
+│       └── main.go         # Minimal bootstrapping (calls internal/cli)
 ├── internal/
 │   ├── broker/             # In-memory pub/sub for long-poll notifications
 │   │   └── broker.go
+│   ├── cli/                # TUI client implementation
+│   │   ├── app.go          # App struct, command handling, poll loop
+│   │   ├── ui.go           # tview-based terminal UI
+│   │   └── api/
+│   │       ├── client.go   # HTTP API client library
+│   │       ├── types.go    # Request/response types
+│   │       └── hashcash.go # Hashcash proof-of-work minting
 │   ├── config/             # Viper-based configuration
 │   │   └── config.go
 │   ├── db/                 # Database access and migrations