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, 5 insertions, 4 deletions

diff --git a/subagents/run.lua b/subagents/run.lua
index 375af91..7fbed67 100644
--- a/subagents/run.lua
+++ b/subagents/run.lua
@@ -1,9 +1,10 @@
 -- The `subagents.run` tool: start one child agent, or continue one, and wait.
 --
--- One call handles both cases. `agent` starts a new child from that profile;
--- `id` continues a child this primary session started earlier. Exactly one is
--- required, and `prompt` is always required — a child cannot see the parent
--- dialogue, so the prompt is the only task context it gets.
+-- One call handles all cases. `agent` starts a new child from that profile;
+-- `system_prompt` starts one without a saved profile; `id` continues a child
+-- this primary session started earlier. Exactly one is required, and `prompt`
+-- is always required — a child cannot see the parent dialogue, so the prompt
+-- is the only task context it gets.
 --
 -- The call blocks until the child settles. Parallelism is ordinary tool
 -- batching: several subagents.run calls emitted in one batch run concurrently