Add background Lua workflows, inline child prompts, and layered config

subagents.lua now starts a workflow on its own coroutine and returns a
session-scoped id immediately, so fan-out continues while the primary keeps
working; completion wakes the primary, and later calls read immutable records
from subagents.workflows. Every ctx:agent takes a workflow-unique name so those
records are addressable. A session_start guidance message tells the primary when
to reach for run vs lua.

Children no longer inherit the primary's system context: a child starts from the
fixed child-role instruction plus its profile, and subagents.run/ctx:agent
accept an inline system_prompt instead of a profile. The now-redundant `agent`
form of subagents.models is gone.

Concurrency defaults to five and is configurable through [subagents]
max_concurrent in any layered config.toml; turn boundaries reap only settled
jobs so background workflows survive, while interrupt and session end cancel.

Config roots come from panto.ext.dirs.layers rather than a hand-rolled XDG
lookup, which picks up the base and git-ignored local layers for both agents/
and workflows/.

TOML workflows tighten up: the subagents.workflow tool takes a discovered name
only (inline `steps` duplicated subagents.lua at less power), an optional
top-level `output` array chooses the reported steps and their order instead of
the terminal set, a step with no workflow input gets no empty input heading, and
a workflow naming an undiscovered agent is rejected at discovery rather than
part-way through a run.

1 files changed, 16 insertions, 20 deletions

diff --git a/subagents/paths.lua b/subagents/paths.lua
index 678e241..8b6d9fa 100644
--- a/subagents/paths.lua
+++ b/subagents/paths.lua
@@ -2,12 +2,13 @@
 --
 -- Two jobs live here:
 --
---   1. Config-layer discovery. `config_roots("agents")` returns the two
+--   1. Config-layer discovery. `config_roots("agents")` returns the
 --      directories profiles (or workflows) are read from, lowest precedence
---      first: `${XDG_CONFIG_HOME:-$HOME/.config}/panto/<name>` and
---      `<cwd>/.panto/<name>`. A root whose base cannot be resolved (no HOME
---      and no XDG_CONFIG_HOME) is simply omitted, so callers must not assume
---      two entries. `walk` then collects `**/*.<suffix>` beneath a root.
+--      first, as `<layer>/<name>` for every layer the host reports in
+--      `panto.ext.dirs.layers` (base, user, project, local). The host owns
+--      that list, so this file never reads HOME or the XDG variables itself;
+--      a host that reports no layers yields no roots. `walk` then collects
+--      `**/*.<suffix>` beneath a root.
 --
 --   2. The child session store. `child_store_dir()` derives
 --      `<session_dir>/subagents/<primary session id>` from the host's
@@ -69,24 +70,19 @@ function M.read_file(path)
     return data
 end
 
--- User layer first, project layer second: later roots shadow earlier ones.
+-- One root per host layer, in the host's order: later roots shadow earlier
+-- ones. A layer the host could not resolve is simply absent from its list.
 function M.config_roots(name)
     local roots = {}
-    local config_home = os.getenv("XDG_CONFIG_HOME")
-    if config_home == nil or config_home == "" then
-        local home = os.getenv("HOME")
-        if home ~= nil and home ~= "" then
-            config_home = home .. "/.config"
-        else
-            config_home = nil
-        end
-    end
-    if config_home ~= nil then
-        roots[#roots + 1] = config_home .. "/panto/" .. name
+    local ok, ext = pcall(host)
+    local layers = ok and type(ext) == "table" and ext.dirs and ext.dirs.layers
+    if type(layers) ~= "table" then
+        return roots
     end
-    local cwd = M.cwd()
-    if cwd ~= nil and cwd ~= "" then
-        roots[#roots + 1] = cwd .. "/.panto/" .. name
+    for _, layer in ipairs(layers) do
+        if type(layer) == "table" and type(layer.dir) == "string" and layer.dir ~= "" then
+            roots[#roots + 1] = layer.dir .. "/" .. name
+        end
     end
     return roots
 end