main: --config/--help arg parsing, vendor on package.path, REPL start

Phase 0 entry point per PHASE0.md §4, §10.

Resolves the §10 config search:
  --config <path>          (explicit; failure if not openable, no fallback)
  $AISH_CONFIG
  ~/.config/aish/config.lua
  ./config.lua

The explicit form now hard-fails instead of silently falling through to
the next candidate — caught in smoke (`--config /nonexistent` was loading
./config.lua).

Pre-pends `./?.lua;./vendor/?.lua` to package.path so `require("dkjson")`
finds vendor/dkjson.lua and project requires resolve from the repo root.
Run from the repo root; cwd-independent resolution lands later.

`--help` prints the usage block. Unrecognized arg exits 2 with a
diagnostic on stderr.

Phase 0 done-criteria (PHASE0.md §2):
  ✓ shell command execution with framed output
  ✓ :meta commands (full §5.2 set)
  ✓ in-memory conversation history with sliding-window eviction
  ✓ codebase layout matches §4 — every module name stable for Phase 1+
  ⏳ live AI exchange — structurally wired; live test deferred per
     issue #12 (broker endpoint hostname not resolvable from noether)

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

main.lua

@@ -2,31 +2,76 @@
 -- Phase 0: arg parsing, config load, REPL start.
 -- See docs/PHASE0.md §4, §10.
 
-local function load_config()
-    -- Resolution order per PHASE0.md §10:
-    --   1. --config <path>   2. $AISH_CONFIG
-    --   3. ~/.config/aish/config.lua   4. ./config.lua
-    -- Phase 0 stub: pick the first existing path; no CLI parsing yet.
+-- Make project modules and the vendored dkjson resolvable from the repo root.
+-- Run aish with the repo root as cwd; PTY-relative resolution lands later.
+package.path = "./?.lua;./vendor/?.lua;" .. package.path
+
+local USAGE = [[
+aish — AI-augmented conversational shell.
+
+Usage: luajit main.lua [--config <path>] [--help]
+
+Config resolution order (PHASE0.md §10):
+  1. --config <path>
+  2. $AISH_CONFIG
+  3. ~/.config/aish/config.lua
+  4. ./config.lua
+]]
+
+local function parse_args(argv)
+    local out = {}
+    local i = 1
+    while i <= #argv do
+        local a = argv[i]
+        if a == "--config" then
+            out.config = argv[i + 1]
+            i = i + 2
+        elseif a == "--help" or a == "-h" then
+            out.help = true
+            i = i + 1
+        else
+            io.stderr:write("aish: unrecognized argument: " .. a .. "\n")
+            os.exit(2)
+        end
+    end
+    return out
+end
+
+local function load_config(opts)
+    -- --config is explicit: use exactly that path or fail. No silent fallback.
+    if opts.config then
+        local f = io.open(opts.config, "r")
+        if not f then
+            error("aish: --config " .. opts.config .. ": cannot open")
+        end
+        f:close()
+        return dofile(opts.config), opts.config
+    end
+
     local home = os.getenv("HOME") or ""
-    local candidates = {
-        os.getenv("AISH_CONFIG"),
-        home .. "/.config/aish/config.lua",
-        "./config.lua",
-    }
+    local candidates = {}
+    local function push(p) if p and p ~= "" then candidates[#candidates + 1] = p end end
+    push(os.getenv("AISH_CONFIG"))
+    push(home .. "/.config/aish/config.lua")
+    push("./config.lua")
+
     for _, path in ipairs(candidates) do
-        if path then
         local f = io.open(path, "r")
         if f then f:close(); return dofile(path), path end
     end
-    end
-    error("aish: no config.lua found in any standard location")
+    error("aish: no config.lua found (tried: "
+          .. table.concat(candidates, ", ") .. ")")
 end
 
-local function main()
-    local config, config_path = load_config()
+local function main(argv)
+    local opts = parse_args(argv or {})
+    if opts.help then io.write(USAGE); return end
+
+    local config, config_path = load_config(opts)
     io.stderr:write(("aish: loaded config from %s\n"):format(config_path))
+
     local repl = require("repl")
     repl.run(config)
 end
 
-main()
+main(arg)