docs: documented how to create custom REPL prompts

docs/REPL-PROMPT.md

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+# Customize REPL Prompt
+
+[//]: # (TODO link to this doc from the main README)
+The prompt you see when you start the Loki REPL can be customized to your liking. This is achieved via the `left_prompt`
+and `right_prompt` settings in the global Loki configuration file:
+
+```yaml
+left_prompt: '{color.red}{model}){color.green}{?session {?agent {agent}>}{session}{?role /}}{!session {?agent {agent}>}}{role}{?rag @{rag}}{color.cyan}{?session )}{!session >}{color.reset} '
+right_prompt: '{color.purple}{?session {?consume_tokens {consume_tokens}({consume_percent}%)}{!consume_tokens {consume_tokens}}}{color.reset}'
+```
+
+The location of the global configuration file differs between systems, so you can use the following command to find your
+global configuration file's location:
+
+```shell
+loki --info | grep 'config_file' | awk '{print $2}'
+```
+
+## Quick Links
+<!--toc:start-->
+- [Syntax](#syntax)
+- [Variables](#variables)
+<!--toc:end-->
+
+## Syntax
+The syntax for the prompts consists of plain text and templates contained in `{...}`. The plain text is 
+printed exactly as given. 
+
+The syntax for the templates `{...}` is as follows:
+
+* `{variable}` - Replaced with the value of `variable`
+* `{?variable <template>}` - Evaluate the `<template>` when `variable` is evaluated to `true`
+* `{!variable <template>}` - Evaluate the `<template>` when `variable` is evaluated to `false`
+
+Where a `<template>` is another expression consisting of plain text and/or more special computations inside `{...}`.
+
+Variables are evaluated to also be "truthy"; that is, if a variable is undefined, it is considered to be the exact same
+as if that variable's value was `false`.
+
+**Example 1: Simple Boolean Usage**
+For the prompt `{?variable yay}{!variable boo}`, if `variable=true`, then the output will be
+```
+yay
+```
+
+And if `variable=false`:
+```
+boo
+```
+
+**Example 2: Nested Expressions**
+For the prompt `{?variable {!variable2 yay}>}`, and assuming
+* `variable=true`
+* `variable2=false`
+the output will be
+```
+yay>
+```
+
+If `variable2=true`, the output will be empty.
+
+If `variable=false`, the output will be empty.
+
+## Variables
+The following variables and output modifiers are available to you when you're creating your prompts:
+
+```yaml
+# Model Variables
+model: openai:gpt-4            # The active model's full name
+client_name: openai            # The name of the client serving the active model
+model_name: gpt-4              # The aliased name of the active model
+max_input_tokens: 4096         # The maximum number of input tokens for the active model
+
+# Configuration Variables
+temperature: 1.0               # The temperature for the active model
+top_p: 0.9                     # The top_p for the active model
+dry_run: true                  # Whether the given command is flagged to be a dry run
+stream: false                  # Whether streaming responses are enabled
+save: true                     # Whether shell history is saved
+wrap: 120                      # The number of characters to allow before wrapping around output to the next line
+
+# Role Variables
+role: code                     # The active role 
+
+# Session Variables
+session: temp                  # The name of the active session
+dirty: false                   # Whether the session settings have been updated but not persisted
+consume_tokens: 200            # The number of tokens consumed
+consume_percent: 1%            # The percentage of tokens consumed to the maximum input tokens
+user_messages_len: 0           # The total number of sent user messages 
+
+# RAG Variables
+rag: temp                      # The name of the active RAG
+
+# Agent Variables
+agent: todo-sh                 # The name of the active agent
+
+# ANSI COLORS
+color.reset:
+color.black:
+color.dark_gray:
+color.red:
+color.light_red:
+color.green:
+color.light_green:
+color.yellow:
+color.light_yellow:
+color.blue:
+color.light_blue:
+color.purple:
+color.light_purple:
+color.magenta:
+color.light_magenta:
+color.cyan:
+color.light_cyan:
+color.white:
+color.light_gray:
+```