Fork hygiene: README banner + upstream sync policy

[axisrow]

Part of #2. The fork is now the living continuation of the project; say so where people look.

Что сделать

1. README: banner at the top — «Actively maintained fork of matt1398/claude-devtools (upstream dormant since 2026-05). This fork carries new development: session-audit CLI, Claude Code plugin, fixes.»
2. README section «Upstream sync policy»: upstream is fetched but not expected to move; local work lives in main (+ feature branches); upstream PRs (fix(parser): parse short model ids without date suffix matt1398/claude-devtools#235) are closed as "carried in fork" once applied locally.
3. CHANGELOG.md: start a fork changelog (entry 1: session-audit CLI, feat(cli): token analytics — session audit + sessions inventory #1).

Acceptance

• Banner + sync section visible on the repo front page
• CHANGELOG.md has the first fork entry