Update project issues: resolve #02-#04, add #08-#10

Mark issues #02 (backtick lexing/parsing), #03 (command execution
runtime), and #04 (argv parsing) as resolved. Add new issues for
configuration (#08), programmable prompt (#09), and interactive
command execution (#10).

issues/09-prompt.md

@@ -0,0 +1,58 @@
+# 09 — Programmable prompt
+
+**Status:** open
+**Blocked by:** #08
+
+Users should be able to customize the shell prompt by defining a Lua function, similar to how other shells use `PS1`/`PROMPT_COMMAND`/`precmd`.
+
+## Design
+
+- The prompt is generated by calling a global Lua function (e.g. `__prompt()`)
+- The function returns a string which is used as the prompt
+- If the function is not defined or errors, fall back to a sensible default
+
+## Default prompt
+
+A built-in default prompt is provided (implemented on the C side) until the user overrides `__prompt()` in their config. The default could be something like:
+
+```
+lush>
+```
+
+or include the current directory:
+
+```
+~/Code/project>
+```
+
+## User customization
+
+Users override the prompt in their config (`~/.config/lush/config`):
+
+```lua
+function __prompt()
+  return $PWD .. "> "
+end
+```
+
+Or with colours/git info/etc:
+
+```lua
+function __prompt()
+  local cwd = $PWD or "?"
+  local branch = `git branch --show-current 2>/dev/null`
+  local git = (branch.code == 0) and " (" .. branch.stdout:gsub("\n","") .. ")" or ""
+  return cwd .. git .. "> "
+end
+```
+
+## Implementation considerations
+
+- The REPL loop (in `lua.c`) calls `__prompt()` before each input line
+- If `__prompt` is not a function or the call fails, use the default C-side prompt
+- The function runs in the normal Lua state so it has access to all globals, env vars, commands, etc.
+
+## Open questions
+
+- Should there be a separate `__continuation_prompt()` for multiline input?
+- Should the prompt function receive any arguments (e.g. exit code of last command)?