repl: @path mention expansion in input lines (closes #7)

Saves the user from manual copy/paste: typing "show me @repl.lua" or
"compare @config.lua and @config.example.lua" auto-expands each mention
to a fenced code block carrying the file contents, language-tagged by
extension, and feeds the composed text to the broker.

Wired on the "ai" branch of the input loop and inside :ask. Meta and
shell branches pass through unchanged — "@foo" in shell context is a
literal program argument; meta commands store text verbatim.

Trigger rule: "@" must follow start-of-string or whitespace — avoids
false positives on email addresses ("user@example.com") and shell
short-options. Path extends to next whitespace.

Other behavior:
  - Language tag derived from extension via a small lookup; unknown
    extensions yield an untagged fence.
  - Files over 32 KB are truncated head/tail (16K + 8K) with a marker.
  - Missing files leave the literal "@path" token in place and emit
    a "[aish] @path: not found" status — non-fatal, lets the user
    correct the path and re-type.
  - Each successful expansion emits "[aish] @path expanded (N bytes
    [, truncated])" so the user sees what was inlined.

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

repl.lua

@@ -13,6 +13,77 @@ local mcp      = require("mcp")
 local safety   = require("safety")
 local json     = require("dkjson")
 
+-- ---------------------------------------------------------------- @-mentions (issue #7)
+-- Triggered when "@" follows start-of-string or whitespace (avoids
+-- false positives on email addresses like user@example.com). Path
+-- runs until next whitespace. The mention is replaced by a fenced
+-- code block carrying the file contents, language-tagged by extension.
+-- Files over MENTION_MAX_BYTES are truncated head+tail with a marker.
+local MENTION_MAX_BYTES = 32 * 1024
+local MENTION_HEAD      = 16 * 1024
+local MENTION_TAIL      = 8 * 1024
+local LANG_BY_EXT = {
+    lua = "lua", py = "python", js = "javascript", ts = "typescript",
+    sh = "bash", c = "c", h = "c", cc = "cpp", cpp = "cpp", hpp = "cpp",
+    rs = "rust", go = "go", java = "java", rb = "ruby", md = "markdown",
+    json = "json", yaml = "yaml", yml = "yaml", toml = "toml",
+    html = "html", css = "css", sql = "sql", xml = "xml",
+}
+local function _lang_of(path)
+    local ext = path:match("%.([%w]+)$")
+    return ext and LANG_BY_EXT[ext:lower()] or ""
+end
+local function _read_truncated(path)
+    local f = io.open(path, "rb")
+    if not f then return nil end
+    local content = f:read("*a") or ""
+    f:close()
+    if #content <= MENTION_MAX_BYTES then return content, false end
+    local head = content:sub(1, MENTION_HEAD)
+    local tail = content:sub(#content - MENTION_TAIL + 1)
+    return head
+        .. ("\n... [%d bytes elided] ...\n"):format(#content - MENTION_HEAD - MENTION_TAIL)
+        .. tail, true
+end
+local function expand_mentions(line, on_status)
+    -- Walk the line; for each "@<path>" preceded by SOL or whitespace,
+    -- attempt to read and substitute. Missing files leave the literal
+    -- token in place + emit a status warning.
+    local out, i = {}, 1
+    while i <= #line do
+        local at_start = (i == 1) or line:sub(i - 1, i - 1):match("%s") ~= nil
+        if at_start and line:sub(i, i) == "@" then
+            local path_end = line:find("%s", i + 1) or (#line + 1)
+            local path = line:sub(i + 1, path_end - 1)
+            if path ~= "" then
+                local content, truncated = _read_truncated(path)
+                if content then
+                    if on_status then
+                        on_status(("@%s expanded (%d bytes%s)"):format(
+                            path, #content, truncated and ", truncated" or ""))
+                    end
+                    out[#out + 1] = ("```%s path=%s\n%s\n```"):format(
+                        _lang_of(path), path, content)
+                    i = path_end
+                else
+                    if on_status then
+                        on_status(("@%s: not found"):format(path))
+                    end
+                    out[#out + 1] = line:sub(i, path_end - 1)
+                    i = path_end
+                end
+            else
+                out[#out + 1] = "@"
+                i = i + 1
+            end
+        else
+            out[#out + 1] = line:sub(i, i)
+            i = i + 1
+        end
+    end
+    return table.concat(out)
+end
+
 local M = {}
 
 local HELP = [[
@@ -24,7 +95,7 @@ Meta commands:
   :models                 list configured models (* = active)
   :history                show conversation turns
   :exec <cmd>             force shell execution
-  :ask <text>             force AI query
+  :ask <text>             force AI query (supports @path expansion)
   :sessions               list session log files
   :save <name>            rename current session log to <name>.jsonl
   :resume <name>          load <name>.jsonl turns into the in-memory context
@@ -783,7 +854,7 @@ function M.run(config)
         ask  = function(args)
             args = (args or ""):match("^%s*(.-)%s*$")
             if args == "" then renderer.status("usage: :ask <text>"); return end
-            ask_ai(args)
+            ask_ai(expand_mentions(args, renderer.status))
         end,
         sessions = function()
             if not sessions_dir then renderer.status("(no history.dir configured)"); return end
@@ -1210,7 +1281,8 @@ function M.run(config)
             elseif kind == "shell" then
                 run_shell(payload)
             else  -- "ai"
-                ask_ai(payload)
+                local expanded = expand_mentions(payload, renderer.status)
+                ask_ai(expanded)
             end
         end
     end