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>

engine/src/test/kotlin/io/flamenet/e2e/X3DHTest.kt

@@ -11,7 +11,7 @@ import kotlin.test.assertTrue
  * The property that matters is agreement: Alice computes a secret from her
  * private keys and Bob's public bundle, Bob computes one from his private keys
  * and what Alice sent, and the two must be identical. A wrong DH order, a
- * swapped argument, or a zero-filled DH4 all still produce 32 plausible bytes —
+ * swapped argument, or a zero-filled DH4 all still produce 32 plausible bytes,
  * they just produce *different* 32 bytes on each side, which is why this checks
  * equality rather than shape.
  */
@@ -71,7 +71,7 @@ class X3DHTest {
     @Test
     fun `using a one-time prekey changes the result`() {
         // Otherwise DH4 is contributing nothing and its absence would be
-        // undetectable — which would make a drained pool silently equivalent to
+        // undetectable, which would make a drained pool silently equivalent to
         // a healthy one.
         val aliceIdentity = X25519.generate()
         val bobIdentity = X25519.generate()