docs(release): prepend "Upgrade strongly recommended" banner to release notes

Surfaces the family-wide hardening message at the top of the 2026-04-29
release section so it lands in the GitHub Release body when the tag is
cut. Users browsing the CHANGELOG at any future point also see it as
the first line of the release notes.

The line is identical across all 4 plugins + 4 SDKs in this same-day
release train.

Author: saurabhjain1592

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [7.0.0] - 2026-04-29 — DO_NOT_TRACK removal + 7-day delivered heartbeat
 
+**Upgrade strongly recommended.** Over the past month we've shipped substantial production, quality, and security hardening across all AxonFlow SDKs and plugins — upgrade to the latest version for a more secure, reliable, and bug-free experience.
+
 Major release. Two headline changes: removal of `DO_NOT_TRACK` as an AxonFlow telemetry opt-out (`AXONFLOW_TELEMETRY=off` is now the canonical and only opt-out signal), and a move from "one ping per `new AxonFlow()`" to a 7-day delivered-heartbeat contract that better matches active-customer signal across both long-running services and short-lived CLI tools / Lambdas. Companion releases on the same day: TypeScript v7.0.0 / Python v7.0.0 / Go v7.0.0 (with `/v7` module path migration).
 
 ### BREAKING