feat(controller): count completed turns by the limit that ended them

[teemow]

Closes #443

Problem

A turn that ends at its Harness's budgetUSD or maxTurns is recorded only in the task's status metadata (stopped_by). Nothing exports it as a metric, so nothing can alert on an Agent whose turns keep ending at a limit: an agent looping, or a limit set too low for its work.

Proposed solution

The controller counts completed turns, kagent.agent.turns (Prometheus kagent_agent_turns_total), labelled:

Label (Prometheus)	Value
gen_ai_agent_name	the Agent's name
k8s_namespace_name	the Agent's namespace
kagent_turn_stopped_by	budget or turns for a turn that ended at that limit; absent for a turn that finished on its own

• Counted in the runtime task store when the runtime settles the task as completed. The new SettleSessionTaskResult returns the task only to the call that published the boundary, so a retried settlement counts once and the count agrees with the stored tasks. Failed, canceled and waiting turns are not counted.
• An Agent's three series are created at zero with its first task, so increase() sees the first completed turn.
• Uses the controller's shared OpenTelemetry provider: on the metrics endpoint the chart's ServiceMonitor scrapes, and in the OTLP export.
• stopped_by moves to go/api/a2a (StoppedByMetadataKey), read by both the controller and the harness runtime.
• Telemetry contract: the metric and the kagent.turn.stopped_by attribute in telemetry/registry, regenerated; docs/architecture/runtime-and-lifecycle.md documents the metric and an alert expression.
• Carried row fork/carried/controller-count-completed-turns-by-the-limit.md; upstream branch upstream/agent-turns-counter, on feat(api): Harness.spec.claude.limits — a budget and a turn cap on every turn of a Claude Code agent kagent-dev/kagent#3020's head.

Acceptance criteria

• A turn ending at budgetUSD counts with stopped_by="budget", one at maxTurns with "turns", a normal turn without the label (unit test TestSettledTurnsAreCountedByStoppedBy, which also reads the Prometheus exposition).
• A retried settlement counts once; failed and waiting turns are not counted.
• Labels are the Agent's name and namespace and stopped_by, nothing per Session or person.
• make semconv-check passes, the generated contract is current, make -C go lint reports 0 issues, the PostgreSQL store tests pass.

[teemow]

Written by an agent: replaced by #448, the same commits on the current giantswarm head (this line is rebase-merged, so a branch behind its base is replaced, not merged up).