Rebuild AgentRunSupervisor on go.gearno.de/kit/worker

The supervisor was a hand-rolled polling, semaphore, and wait-group
loop predating the project's adoption of the shared worker kit. Two
sibling workers in pkg/probo already use the kit, and go-worker.md
documents it as the project convention.

This commit introduces agentRunHandler, which implements
worker.Handler[coredata.AgentRun] and worker.StaleRecoverer, and
reduces AgentRunSupervisor to a thin wrapper that owns the handler
plus a worker.Worker and bridges ctx cancellation into a handler-
level shutdown broadcast via context.AfterFunc. The agent stop
channel is now closed by a per-Process forwarder goroutine when the
broadcast fires, so in-flight runs checkpoint at the next turn
boundary and drain through wg.Wait before Run returns.

The stop_requested column, struct field, supporting SQL, and the
LoadRunningStopRequestedIDs function are removed end-to-end. None
of it was ever wired to an external surface; it existed purely to
let the supervisor find runs the operator wanted to halt. With the
kit handling the polling cadence and the AfterFunc bridging
shutdown, per-row flagging is dead weight.

The supervisor's public API (NewAgentRunSupervisor, Run, the With*
option helpers, and the error sentinels) stays intact so probod.go
needs no change. The integration test now triggers stop by
cancelling the supervisor context, which is the actual production
path through SIGTERM rather than a synthetic DB flag. Prometheus
counters and OTel spans labelled worker="agent-run-supervisor"
come for free.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/coredata/migrations/20260424T120000Z.sql

@@ -22,7 +22,6 @@ CREATE TABLE agent_runs (
     input_messages  JSONB NOT NULL,
     result          JSONB,
     error_message   TEXT,
-    stop_requested  BOOLEAN NOT NULL DEFAULT FALSE,
     started_at      TIMESTAMPTZ,
     lease_owner     TEXT,
     lease_expires_at TIMESTAMPTZ,