Remove machine-writing tells from comments and docs

Emoji markers, markdown bold inside code comments, and em-dashes are
gone from source comments, string literals, scripts, Gradle files and
the markdown docs. Counted before and after:

  emoji markers   2680 -> 1 (the emoji picker's own list, which is the feature)
  em-dashes       2490 -> 9 (table placeholders and fenced code)

Two warning glyphs in user-facing strings in ChatScreens.kt are removed
as well; the sentences say the same thing without them. Two backtick
test names carried an em-dash and now carry a comma.

Nothing else changed. 1208 tests pass, counted from the result XML.
The three obfs4 interop tests are opted out with OBFS4_INTEROP_OPTOUT=1
for this run, as their own message instructs when no bridge is set up.

CONTRIBUTING gets a four-line rule so it does not grow back.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

interop/build.gradle.kts

@@ -32,5 +32,5 @@ tasks.register<JavaExec>("peer") {
 }
 
 // No `jvmTarget` pin here, unlike `:engine`. This never runs on Android, so
-// bytecode level 17 buys nothing — and pinning it while `compileJava` follows
+// bytecode level 17 buys nothing, and pinning it while `compileJava` follows
 // the local JDK is how the two tasks end up disagreeing and failing the build.