docs(deploy): example Kubernetes manifests for Prometheus scrape, recording rules, alert rules

[cjimti]

Part of #459.

Problem

There is no operator-facing documentation for wiring a Prometheus scraper to
the platform. Each install reinvents annotations, rules, and alerts. No Helm,
plain Kubernetes manifests only.

Acceptance criteria

New directory deployments/observability/ containing:

• pod-annotations.yaml - example Deployment patch showing
  prometheus.io/scrape: "true", prometheus.io/port: "9090",
  prometheus.io/path: "/metrics".
• recording-rules.yaml - ConfigMap with the starter recording rules.
• alert-rules.yaml - ConfigMap with the starter alert rules.
• README.md - how to apply, where to mount, how to confirm Prometheus is
  actually scraping (check up{job=...} query).

Plus:

• Plain Kubernetes resources only. NO Helm chart. NO HelmRelease. Kustomize is
  optional; the raw files must work standalone with kubectl apply -f.
• Recording rule naming follows Prometheus best practice
  level:metric_name:operations style.
• All rules pass promtool check rules.
• Underlying metrics documented in docs/observability.md.
• Per CLAUDE.md doc-update rule: docs/llms.txt and docs/llms-full.txt also updated.

Starter recording rules

• mcp:tool_call_duration:p95_5m
• mcp:tool_call_error_rate:5m grouped by tool
• apigateway:inbound_duration:p50_5m
• apigateway:inbound_duration:p95_5m
• apigateway:inbound_duration:p99_5m
• apigateway:inbound_error_rate:5m grouped by connection
• apigateway:outbound_duration:p95_5m grouped by connection

Starter alert rules

• High 5xx rate per connection sustained over 5m.
• Inbound p95 latency regression vs trailing 1h baseline.
• Auth failure spike of any kind over 5m.
• DB pool wait_count growing (saturation indicator).
• up{job="mcp-data-platform"} == 0 (pod down).

Depends on

• Inbound API gateway metrics and toolkit metrics sub-issues, so the rules can
  reference real metric names.

Files

• deployments/observability/*
• docs/observability.md
• docs/llms.txt
• docs/llms-full.txt