docs: rewrite README for coherence and accuracy as of v0.4.1

Consolidated scattered/patchwork edits from today's releases into a
single coherent Features section, merged the "why gray" and backoff
explanations (previously split across two places) into one Reliability
section, and corrected a real inaccuracy: the old Privacy section
claimed only one registry location was ever written, which has been
stale since v0.2.0/v0.3.0 -- it now precisely lists all three
(Run key, the app's own settings key, and the NotifyIconSettings
IsPromoted flag), matching src/registry.rs exactly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>