docs: README rewrite, fork delta, runbooks, design notes, AGENTS.md

Fresh README replaces the upstream supabase-only quickstart with the
Azure-fork story: what's added, where to find local-stack docs, where
to find azure-prereqs.md, and the explicit note that one-click
deployment lives in a separate marketplace package.

Adds the runtime config / local-first / Entra runbooks and the
provider-boundary design notes.  fork-delta.md is rewritten for the
OSS audience - drops the internal three-tier publication framing and
just describes what diverges from upstream.

Adds the application-layer subset of the original azure-migration
issue archive (the deployment-specific issues stay in the separate
deploy repo).  Sanitised - concrete tenant identifiers, FQDNs, and
resource names replaced with placeholders.

  * README.md, AGENTS.md
  * docs/azure-prereqs.md             - provision-it-yourself prose
  * docs/fork-delta.md                 - file-by-file divergence list
  * docs/runbook-local-stack.md
  * docs/runbook-entra-local-auth.md
  * docs/auth-provider-selection-flow.md
  * docs/local-first-upstream-strategy.md
  * docs/agent-handoff.md
  * docs/{Postgres,storage,email,entraId}/* - design deep-dives
  * docs/AzureMove/azure-migration-proposal.md
  * docs/issues/azure-migration/{004,006.1,006.2,008,008.0,009,010,
                                 011,012,013,014,017-022,024-028,
                                 030-runtime-config-endpoint,031-supabase-placeholder-removal,
                                 032-retire-env-production}.md
  * routes/install.ts: removed lingering hardcoded KV name from a
    save-confirmation HTML string surfaced via /install.