docs(release): README polish + v1.5.0 (#1400)

- Hero: larger theme-aware standalone Rust logo (new assets/rust-logo{,-dark}.svg
  — gear only, no tile card; <picture> swaps by GitHub theme), tagline text
  trimmed to 'Kernel powered by Rust'
- 'Built for speed' section: removed the floated language-tile logo (its baked-in
  paper card rendered as an odd box on dark theme and pushed the text)
- Removed the '1.0 Released!' banner and the Star History section (+ its
  Contents entry)
- package.json → 1.5.0 for the Rust-engine release

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@colbymchenry/codegraph",
-  "version": "1.4.1",
+  "version": "1.5.0",
   "description": "Supercharge AI coding agents with semantic code intelligence — surgical context, fewer tool calls, faster answers. 100% local.",
   "repository": {
     "type": "git",