docs: restructure along Diátaxis — maturity tiers, non-goals, tutorial, how-tos

[ThomasK33]

Restructure user-facing documentation along Diátaxis and make the public story honest and tiered.

Scope

• README: add an adapter maturity tier table (supported = production-grade with hardening tests + docs; experimental = honest label, no promises), applied honestly to Slack / Linear / (Teams: spike, see feat(adapters/msteams): land Microsoft Teams adapter as experimental (needs live-tenant validation) #6). Add an explicit Non-goals section (streaming token transport in core, LLM routing/prompt orchestration, generative UI card DSL, RAG/embeddings, durable transcript persistence in chat.State, app-user auth orchestration) with one-line rationales pointing at ADRs.
• docs/: Diátaxis skeleton — tutorials/ (zero → running Slack bot in under 30 minutes, actually verified), how-to/ (multi-tenant install, choosing a state backend, deferred dispatch, interactive components, slash commands, Linear agent sessions), reference (pkg.go.dev pointers, doc.go coverage), explanation (index over CONTEXT.md + ADRs).
• docs/agents/issue-tracker.md: GitHub issues are now the public source of truth; .scratch/ demoted to internal working notes.

Acceptance

• A newcomer can go from clone to running Slack bot following only the tutorial.
• Every claim in README matches implemented reality (verified against code, not aspiration).
• No dead links; docs index reachable from README.

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