chore(changelog): hygiene scrub for 2026-05-09 release [skip-runtime-e2e]

Sweep CHANGELOG.md to remove engineering-context references that should
not appear on the customer-facing release surface, per the project's
CHANGELOG hygiene rule:

- ADR references
- Issue / Epic / PR numbers
- Cross-repo references (e.g. axonflow-enterprise#NNNN)
- Sister-PR / internal-session cross-references

Also re-dates the v8.0 / v0.2 release header(s) from 2026-05-08 →
2026-05-09 to match the actual ship day.

Docs-only — no code, public API, or wire-schema changes.

Signed-off-by: Saurabh Jain <saurabhjain1592@gmail.com>

Author: saurabhjain1592