docs: align V07 repository guidance

[sufyanaser]

What changed

• aligns the README with the published V07 / v7.0.0 release;
• documents the installer, first update-enabled release boundary, local data, and validation commands;
• adds security, contribution, conduct, issue-form, and pull-request guidance.

Why

The repository README still presented V06 while V07 and its updater assets are already public. The repository also lacked explicit security and contribution workflows for local notes, OAuth credentials, RTL behavior, and persistence-sensitive changes.

Validation

• YAML parsing for every issue form and workflow
• npm ci
• npm run lint
• npm run typecheck
• npm test — 64 passed
• npm run build

Known baseline

npm audit reports two transitive advisories (one moderate, one high). They are not introduced by this documentation/community change and should be handled in a separate dependency PR with regression testing.