Validation-pass fixes; DESIGN.md describes the shipped seam

jobs: a child whose wake pipe cannot be armed is refused up front instead of
started into a state nothing can wake (the luv-less drain loop survives only
for hosts without a loop); a closing child keeps its concurrency slot and
its id's exclusivity until the pump actually exits, so teardown can no
longer over-admit new children or let two turns share one session file.
Resume-metadata reads honor the plain-error contract on a malformed store.

workflow: the built-in schema subset validator is the only validator — the
jsonschema probe made behavior depend on an undeclared rock (see rockspec:
that dependency is deliberately rejected); dead exports and the unreachable
half of the structured-output guard are gone, keeping the empty-arguments
provider case.

DESIGN.md's seam sections now describe the shipped division: binding-level
async jobs and tool control, host-level resolve_model/ExtHost/turn events/
component handles, rock-level policy; protocol bodies run to completion on
the loop thread and cannot yield, cancellation is scoped to the stream that
opened it, and a child's compaction leaves protocol sessions alone.

1 files changed, 16 insertions, 31 deletions

diff --git a/subagents/workflow.lua b/subagents/workflow.lua
index b1d3e46..3cf58d9 100644
--- a/subagents/workflow.lua
+++ b/subagents/workflow.lua
@@ -40,12 +40,14 @@
 -- * Handles the callback never awaited are awaited ("all") after it returns,
 --   purely so no child is orphaned; those results are discarded.
 -- * Structured output is decoded from result.structured_json and validated
---   against output.schema. Validation prefers the `jsonschema` rock and falls
---   back to the small built-in subset validator below when it is absent --
---   that rock pulls in lrexlib-pcre, which needs a system PCRE and fails to
---   build on stock macOS, and a failed rock install would otherwise take the
---   whole extension down silently. A validation failure turns the result into
---   status "failed"; it is never reported as a successful structured result.
+--   against output.schema by the one validator below: the JSON Schema subset
+--   a child's output tool actually uses, ignoring keywords it does not know.
+--   There is deliberately no second, rock-dependent path -- `jsonschema` needs
+--   lrexlib-pcre and a system PCRE that stock macOS lacks, so it is not a
+--   declared dependency, and a validator picked by whether a rock happens to
+--   be installed would make the same output pass here and fail there. A
+--   validation failure turns the result into status "failed"; it is never
+--   reported as a successful structured result.
 -- * The host seam is reached through `require("panto").ext` at call time, not
 --   aliased at load time, matching subagents/spawn.lua so a test can install a
 --   fake `panto` module before the first call rather than before the require.
@@ -58,8 +60,6 @@ local M = {}
 
 local workflow_mt = { __name = "subagents.workflow" }
 
-M.workflow_mt = workflow_mt
-
 -- ---------------------------------------------------------------------------
 -- Host seam access
 -- ---------------------------------------------------------------------------
@@ -117,16 +117,15 @@ local function json_encode(value)
     return tostring(value)
 end
 
-M.json_decode = json_decode
 M.json_encode = json_encode
 
 -- ---------------------------------------------------------------------------
 -- Schema validation
 -- ---------------------------------------------------------------------------
 
--- Built-in fallback validator: the JSON Schema subset that structured child
--- output actually uses. Anything it does not understand is ignored rather than
--- rejected, so an unrecognized keyword never fails a legitimate result.
+-- The validator: the JSON Schema subset that structured child output actually
+-- uses. Anything it does not understand is ignored rather than rejected, so an
+-- unrecognized keyword never fails a legitimate result.
 local function is_array_like(value)
     local count = 0
     for key in pairs(value) do
@@ -259,23 +258,6 @@ local function check_schema(value, schema, path)
     return true
 end
 
-local function validator_for(schema)
-    local ok, jsonschema = pcall(require, "jsonschema")
-    if ok and type(jsonschema) == "table" and jsonschema.generate_validator then
-        local generated_ok, generated = pcall(jsonschema.generate_validator, schema)
-        if generated_ok and type(generated) == "function" then
-            return generated
-        end
-    end
-    return function(value)
-        return check_schema(value, schema, "output")
-    end
-end
-
-M.validate = function(value, schema)
-    return validator_for(schema)(value)
-end
-
 -- ---------------------------------------------------------------------------
 -- Result shaping
 -- ---------------------------------------------------------------------------
@@ -312,8 +294,11 @@ local function shape_result(result, handle)
         return shaped
     end
 
+    -- An empty tool input is a real provider case (a chat-style provider
+    -- finalizes an argument-less call with ""), and it is not a validation
+    -- failure: nothing was produced to validate.
     local raw = shaped.structured_json
-    if type(raw) ~= "string" or raw == "" then
+    if raw == nil or raw == "" then
         return fail(shaped, "structured output missing: the child produced no structured result")
     end
 
@@ -322,7 +307,7 @@ local function shape_result(result, handle)
         return fail(shaped, "structured output failed validation: " .. tostring(decoded))
     end
 
-    local valid, message = validator_for(schema)(decoded)
+    local valid, message = check_schema(decoded, schema, "output")
     if not valid then
         return fail(shaped, "structured output failed validation: " .. tostring(message or "schema mismatch"))
     end