docs: 0.8 release docs

[artberger]

Summary

Adds 7 new concept documentation pages for the 0.8 release. This is a corrected version of #321 — all content has been validated against the actual code in kagent-dev/kagent.

New pages

• Tools Ecosystem — Complete catalog of all built-in MCP tools (K8s, Helm, Istio, Argo, Cilium, Grafana MCP)
• Human-in-the-Loop — Tool approval workflows and Ask User tool
• Agent Memory — Vector-backed long-term memory configuration
• Prompt Templates — Go template syntax with ConfigMap data sources
• Multi-Runtime Support — Go vs Python ADK runtimes and BYO agents
• Git-Based Skills — Loading skills from Git repositories
• Context Management — Event compaction and optional summarization

Fixes vs original PR #321

CRD field structures corrected:

• Memory config uses modelConfig (string ref to ModelConfig CR), not embeddingProvider sub-object. No enabled flag — presence of the field enables memory.
• Prompt template dataSources uses TypedLocalReference (kind/apiGroup/name), not configMapRef.name
• Context management path is context.compaction, not context.eventsCompaction.enabled
• OCI skill refs is a []string, not objects with image field
• BYO agent image is at spec.byo.deployment.image, not spec.byo.image

Tools Ecosystem rewritten:

• All Kubernetes tool names corrected: kubectl_* → k8s_* (e.g., k8s_get_resources, k8s_describe_resource)
• Helm: helm_list → helm_list_releases, added helm_get_release, removed fabricated helm_install
• Istio: corrected names (e.g., istio_install_istio, istio_generate_manifest, istio_analyze_cluster_configuration)
• Argo Rollouts: removed 4 fabricated tools, kept only argo_verify_argo_rollouts_controller_install
• Cilium: organized by agent type (Manager/Debug/Policy) with all 40 verified tool names
• Prometheus/Grafana: replaced fabricated standalone tools with actual Grafana MCP server tools
• Removed fabricated utility tools (datetime_format, shell_execute, etc.)
• Removed fabricated kagentTools.readOnly Helm value claim
• Removed fabricated Prometheus metrics claim

Other fixes:

• Memory API routes corrected (uses query params, not path params)
• Go ADK startup time: ~1 second (not ~2)
• HITL architecture: removed incorrect "preprocessor" terminology
• Context management: clarified summarization requires explicit summarizer config
• Prompt templates: clarified that systemMessageFrom supports Secrets (dataSources are ConfigMap-only)

Validation method

Every claim was verified by searching the kagent-dev/kagent source code at commit 2dcdd113 on main. Tool names were extracted from Helm agent templates (helm/agents/*/templates/agent.yaml). CRD structures were verified against go/api/v1alpha2/agent_types.go.