router: classify(line, config) -> (kind, payload)

Phase 0 implementation per PHASE0.md §5.

Pure function. Three kinds:
  "meta"  — line starts with ":", payload is the rest
  "shell" — line starts with "$" (override, $ stripped), OR first word
            is in config.shell.known_commands, OR first word is
            path-like (`./`, `../`, `/`)
  "ai"    — everything else (including empty / whitespace-only; the
            repl loop skips empty payloads before dispatching)

Path-like detection is deliberately conservative in Phase 0: anchored
prefixes only, no quoted-path or shell-glob handling. Q4 in §13 tracks
multi-command CMD: blocks; this router doesn't see those (it only
classifies user input lines, not assistant output).

Smoke covers all branches plus a nil-config fallthrough.

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

router.lua

@@ -1,14 +1,63 @@
 -- router.lua — task classifier: meta / shell / AI.
 -- See docs/PHASE0.md §5.
+--
+-- Pure function. Takes (line, config). Returns (kind, payload).
+--   kind   : "meta" | "shell" | "ai"
+--   payload: the (possibly stripped) line that the dispatcher should act on.
+--
+-- Empty / whitespace-only lines are returned as ("ai", ""); the repl loop
+-- skips them before dispatching.
 
 local M = {}
 
--- Classify an input line.
+local function trim(s)
--- Returns one of: "meta", "shell", "ai" plus the (possibly stripped) payload.
+    return (s:gsub("^%s+", ""):gsub("%s+$", ""))
-function M.classify(line, config)
+end
-    error("router.classify: not implemented (Phase 0 pending)")
+
+local function first_word(s)
+    return s:match("^(%S+)") or ""
+end
+
+local function known_commands_set(config)
+    local set = {}
+    local list = config and config.shell and config.shell.known_commands or {}
+    for _, c in ipairs(list) do set[c] = true end
+    return set
+end
+
+-- §5.1 path-like: ./foo, ../foo, /usr/bin/foo. Quoted/escaped paths are
+-- intentionally out of scope in Phase 0.
+local function path_like(token)
+    return token:sub(1, 1)  == "/"
+        or token:sub(1, 2)  == "./"
+        or token:sub(1, 3)  == "../"
+end
+
+function M.classify(line, config)
+    line = trim(line or "")
+    if line == "" then return "ai", "" end
+
+    -- meta: ":" prefix
+    if line:sub(1, 1) == ":" then
+        return "meta", line:sub(2)
+    end
+
+    -- shell explicit override: "$" prefix
+    if line:sub(1, 1) == "$" then
+        return "shell", trim(line:sub(2))
+    end
+
+    local first = first_word(line)
+    local known = known_commands_set(config)
+
+    -- known-command allowlist
+    if known[first] then return "shell", line end
+
+    -- path-like first token
+    if path_like(first) then return "shell", line end
+
+    -- everything else -> AI
+    return "ai", line
 end
 
--- Default known-command allowlist seeds the heuristic in §5.1.
--- Final list is config.shell.known_commands at runtime.
 return M