Support configurable base_url and optional API key, fixes #1

[quentin@void ~]$ yo where is my yosh config?
Your yosh config is at ~/.yoconf. It's a simple text file where you can set things like:

- provider - Which LLM provider to use (anthropic, openai)
- key - Your API key
- model - What model to use
- chat_prefix / chat_reset - How chat/error output is formatted
- history_limit / token_budget - Session memory limits
- scrollback_enabled / scrollback_bytes / scrollback_lines - Terminal output capture
- server_web - Enable/disable web search

If you just want a quick setup without the config file, you can use the key files directly: ~/.anthropickey (for Anthropic) or ~/.openaikey (for OpenAI) — they need mode 0600 and contain just your API key on a single line.
[quentin@void ~]$ cat ~/.yoconf
provider openai
model glm-4.7-flash
base_url http://localhost:11434
key dummy

[quentin@void ~]$ yo what is my SHELL?
Check SHELL environment variable and current process name
[quentin@void ~]$ echo $SHELL && ps -p $$ -o comm=
/bin/bash
yosh
Your shell is yosh! It's running as the current process (you can see yosh in the ps output).

Your $SHELL environment variable points to /bin/bash, but you're actively using yosh as your interactive shell. Yosh is a bash-compatible shell that's based on bash 5.2.32.
[quentin@void ~]$ yo how do I add yosh as my default shell?
Check if chsh is available and switch shell to yosh
[quentin@void ~]$ which yosh && chsh -s $(which yosh)
/usr/local/bin/yosh
Changing shell for quentin.
Password:
chsh: "/usr/local/bin/yosh" is not listed in /etc/shells.
Use chsh -l to see list.
Show available shells in /etc/shells
[quentin@void ~]$ sudo chsh -l
Password:
/usr/bin/bash
/bin/bash
/usr/bin/sh
/usr/bin/dash
/bin/sh
/bin/dash
Add yosh to /etc/shells so chsh can use it
[quentin@void ~]$ echo "/usr/local/bin/yosh" | sudo tee -a /etc/shells
/usr/local/bin/yosh
Switch default shell to yosh
[quentin@void ~]$ chsh -s /usr/local/bin/yosh
Changing shell for quentin.
Password:
Shell changed.
Verify yosh is now the default shell
[quentin@void ~]$ echo $SHELL && ps -p $$ -o comm=
/bin/bash
yosh