[docs] Swap social preview and title to tacet

Replaces the hark social card with the tacet one: the source image is
the original card edited in place, so the waveform bars, gradient,
glow, and block cursor are carried over unchanged and only the
wordmark differs. Sized to a true 1200x630 this time; the hark card
was 1280x640 and cropped on Twitter and LinkedIn.

Also repoints the CI badge, which still hardcoded the old repository
path and would have started 404ing once the redirect lapses.

The body of the README still documents ~/.config/hark and the other
current identifiers. That is deliberate and accurate — those paths do
not change until the code rename in #19 lands.

docs/hark-social.png is kept for now so previously scraped Open Graph
URLs do not break; it goes with #19.

Solves: project identity after the hark -> tacet rename
Tests: none (docs); rendered image verified at 1200x630

Author: drycode

README.md

@@ -1,8 +1,8 @@
-# hark
+# tacet
 
-[![CI](https://github.com/DRYCodeWorks/hark/actions/workflows/ci.yml/badge.svg)](https://github.com/DRYCodeWorks/hark/actions/workflows/ci.yml)
+[![CI](https://github.com/DRYCodeWorks/tacet/actions/workflows/ci.yml/badge.svg)](https://github.com/DRYCodeWorks/tacet/actions/workflows/ci.yml)
 
-![hark — an audio waveform resolving into the word "hark" followed by a terminal cursor](docs/hark-social.png)
+![tacet — an audio waveform resolving into the word "tacet" followed by a terminal cursor](docs/tacet-social.png)
 
 Push-to-talk dictation into whatever has focus — most usefully a terminal
 session reached over SSH or mosh, but really anywhere: Slack, a browser, any