repl: user-defined skills loader (closes #2)

PHASE0.md §5.2 froze the meta-command set at compile time. Skills let
the user package repeatable workflows (project queries, prompt
templates, audit routines) without forking aish.

Discovery: scan ~/.config/aish/skills/*.lua at startup (or whatever
$AISH_SKILLS_DIR points at — used both by users with non-XDG layouts
and by CI). Each module exports:

    return {
        name        = "<meta-cmd-name>",     -- must match [%w_-]+
        description = "<one-line>",          -- shown by :skills
        run         = function(args, h) ... end,
    }

Helpers passed to run():
    h.ask(text)   — same path as :ask (with @path expansion)
    h.status(s)   — emit "[aish] s"
    h.exec(cmd)   — run a shell command (subject to plan_mode, hooks)
    h.model()     — current active model name
    h.ctx         — raw Context object (advanced)
    h.config      — the loaded config table

Validation rejects modules that miss name/run, use whitespace in the
name, or collide with an existing meta command (built-in or earlier
skill). Each rejection emits a status line so the user sees why a
skill didn't appear.

New meta command :skills lists what's loaded (sorted, with description).

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

repl.lua

@@ -136,6 +136,7 @@ Meta commands:
   :route classes          show current class → model mapping
   :route check <text>     report which class <text> would route to (debug)
   :fallback on/off        toggle cloud retry when local transport fails
+  :skills                 list user-defined skills loaded from ~/.config/aish/skills/
   :help                   this message
 ]]
 
@@ -1340,6 +1341,73 @@ function M.run(config)
         help = function() io.write(HELP) end,
     }
 
+    -- Issue #2: user-defined skills loader. Scan ~/.config/aish/skills/
+    -- (or $AISH_SKILLS_DIR) for *.lua modules. Each module returns
+    -- { name = "<meta-cmd-name>", description = "...", run = function(args, h) end }
+    -- and gets registered as a meta command :<name>. Helpers passed to run():
+    --   h.ask(text)     -- send text as an ai-kind prompt (same path as :ask)
+    --   h.status(s)     -- emit a [aish] status line
+    --   h.exec(cmd)     -- run a shell command (subject to plan/hooks)
+    --   h.model()       -- current active model name
+    --   h.ctx           -- raw context object (advanced)
+    --   h.config        -- raw config table
+    local skills = {}  -- { [name] = {description=, run=} }
+    local function load_skills()
+        local dir = os.getenv("AISH_SKILLS_DIR")
+                    or ((os.getenv("HOME") or ".") .. "/.config/aish/skills")
+        local pipe = io.popen(("ls -1 %q/*.lua 2>/dev/null"):format(dir))
+        if not pipe then return end
+        for path in pipe:lines() do
+            local ok, mod = pcall(dofile, path)
+            if not ok then
+                renderer.status(("skill load failed: %s: %s")
+                                :format(path, tostring(mod)))
+            elseif type(mod) ~= "table"
+                or type(mod.name) ~= "string"
+                or type(mod.run) ~= "function"
+                or not mod.name:match("^[%w_-]+$")
+            then
+                renderer.status(("skill %s: invalid module (need {name, run})")
+                                :format(path))
+            elseif meta[mod.name] or skills[mod.name] then
+                renderer.status(("skill %s: name '%s' already in use")
+                                :format(path, mod.name))
+            else
+                skills[mod.name] = {
+                    description = mod.description or "",
+                    run         = mod.run,
+                }
+                local helpers = {
+                    ask    = function(t) ask_ai(expand_mentions(t or "", renderer.status)) end,
+                    status = renderer.status,
+                    exec   = run_shell,
+                    model  = function() return active_name end,
+                    ctx    = ctx,
+                    config = config,
+                }
+                meta[mod.name] = function(args)
+                    local sk_ok, sk_err = pcall(mod.run, args or "", helpers)
+                    if not sk_ok then
+                        renderer.status(("skill %s failed: %s")
+                                        :format(mod.name, tostring(sk_err)))
+                    end
+                end
+            end
+        end
+        pipe:close()
+    end
+    meta.skills = function()
+        local names = {}
+        for n, _ in pairs(skills) do names[#names + 1] = n end
+        table.sort(names)
+        if #names == 0 then renderer.status("(no skills loaded)"); return end
+        renderer.status(("skills (%d):"):format(#names))
+        for _, n in ipairs(names) do
+            io.write(("  :%-16s %s\n"):format(n, skills[n].description))
+        end
+    end
+    load_skills()
+
     -- Main loop.
     while true do
         local line = rl.readline(prompt())