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, 11 insertions, 3 deletions

diff --git a/subagents/progress.lua b/subagents/progress.lua
index 03fe62a..d88e239 100644
--- a/subagents/progress.lua
+++ b/subagents/progress.lua
@@ -326,8 +326,9 @@ end
 function M.settle(event)
     local key = type(event) == "table" and (event.id or event.tool_call_id) or nil
     local board = key and boards[key]
-    if not board then return end
-    set_board_pinned(board, false)
+    if board then set_board_pinned(board, false) end
+    local co = coroutine.running()
+    if co and bound[co] == key then bound[co] = nil end
 end
 
 function M.collapse(event)
@@ -347,6 +348,12 @@ function M.bind(context)
     if co then bound[co] = key end
 end
 
+function M.bind_coroutine(co, tool_call_id)
+    if type(co) ~= "thread" then return end
+    if type(tool_call_id) ~= "string" or tool_call_id == "" then tool_call_id = nil end
+    bound[co] = tool_call_id
+end
+
 -- The outer tool call is durable child-turn metadata, not model-visible output.
 -- Expose it to the spawn seam without making the board state global: workflow
 -- callbacks run on the same coroutine as their owning tool handler.
@@ -723,7 +730,8 @@ function M.reset()
         set_board_pinned(board, false)
     end
     prune_boards()
-    bound = setmetatable({}, { __mode = "k" })
+    -- `bound` has weak coroutine keys. Foreground handlers clear themselves in
+    -- settle(); background workflow bindings must survive turn boundaries.
 end
 
 return M