repl: configurable prompt template via config.shell.prompt (closes #10)

At-a-glance situational awareness: see the active model, context fill,
mode flags, and cwd in the prompt itself — prevents "wait, am I still
in plan mode?" surprises.

Example config:

    shell = {
        prompt = "[{model} {ctx_used}/{ctx_max}t T{turn} {mode}] {cwd_short} > ",
    }

Variables (substituted via {name}):
  {model}        active preset name
  {ctx_used}     char/4 token heuristic (Phase 0 §8; accurate is Q1)
  {ctx_max}      config.context.token_budget
  {turn}         #ctx.turns
  {cwd}          libc.getcwd() (chdir-aware; PWD env may drift)
  {cwd_short}    cwd with $HOME -> ~
  {last_status}  last exec exit code, "" if none yet
  {mode}         "norris" | "plan" | "normal"

Default behavior unchanged when shell.prompt is unset — keeps the
"[aish:<model>]>" form with norris ⚡ and plan markers.

Side wiring:
  - ffi/libc.lua gains getcwd() (chdir() doesn't update PWD).
  - run_shell records exit code into last_exec_code for {last_status}.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.lua

@@ -42,6 +42,14 @@ return {
         },
         capture_output = true,    -- inject exec output into context
         confirm_cmd    = true,    -- prompt before executing CMD: suggestions
+
+        -- Issue #10: prompt template. When set, replaces the default
+        -- "[aish:<model>]> " prompt. Variables (substituted via {name}):
+        --   {model}  {ctx_used}  {ctx_max}  {turn}
+        --   {cwd}    {cwd_short} (cwd with $HOME -> ~)
+        --   {last_status} (last exec exit code, empty if none yet)
+        --   {mode}   (norris / plan / normal)
+        -- prompt = "[{model} {ctx_used}/{ctx_max}t T{turn} {mode}] {cwd_short} > ",
     },
 
     context = {