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>

repl.lua

@@ -449,7 +449,40 @@ function M.run(config)
         if session then session:append(turn) end
     end
 
+    -- Issue #10: configurable prompt template. When config.shell.prompt is
+    -- set, substitute {model}/{ctx_used}/{ctx_max}/{turn}/{cwd}/{cwd_short}
+    -- /{last_status}/{mode}. Otherwise fall back to the default with the
+    -- norris ⚡ + plan markers.
+    local libc = require("ffi.libc")
+    local last_exec_code = nil
+    local function _cwd_short()
+        local c = libc.getcwd() or os.getenv("PWD") or "?"
+        local home = os.getenv("HOME")
+        if home and c:sub(1, #home) == home then
+            c = "~" .. c:sub(#home + 1)
+        end
+        return c
+    end
+    local function _mode()
+        if ctx.norris_active then return "norris" end
+        if plan_mode             then return "plan"   end
+        return "normal"
+    end
     local function prompt()
+        local tmpl = config.shell and config.shell.prompt
+        if tmpl then
+            local vars = {
+                model       = active_name,
+                ctx_used    = tostring(ctx:estimate_tokens()),
+                ctx_max     = tostring(ctx.token_budget),
+                turn        = tostring(#ctx.turns),
+                cwd         = libc.getcwd() or "?",
+                cwd_short   = _cwd_short(),
+                last_status = last_exec_code and tostring(last_exec_code) or "",
+                mode        = _mode(),
+            }
+            return (tmpl:gsub("{(%w+)}", function(k) return vars[k] or "" end))
+        end
         if ctx.norris_active then
             return ("[aish:%s \xE2\x9A\xA1]> "):format(active_name)
         end
@@ -544,6 +577,7 @@ function M.run(config)
         end
         renderer.exec_begin()
         local out, code = executor.exec(cmd)
+        last_exec_code = code
         renderer.exec_end(code)
         if config.shell and config.shell.capture_output then
             ctx:append_exec_output(out)