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/AccountKdfTest.kt

@@ -16,8 +16,8 @@ import kotlin.test.assertTrue
  * and an empty one, which is where a `CharArray` API and a UTF-8 wire format
  * quietly disagree.
  *
- * ⚠️ Passing here says nothing about a phone. The provider on a device is
- * Conscrypt, not the JDK's, and this engine already has one case — X25519 —
+ * Passing here says nothing about a phone. The provider on a device is
+ * Conscrypt, not the JDK's, and this engine already has one case. X25519,
  * that passed twenty-one JVM tests and threw on the first tap on real hardware.
  * The on-device suite in `:engine-android` is what settles Android.
  */
@@ -77,7 +77,7 @@ class AccountKdfTest {
         val identity = AccountKdf.identityKey(pass, salt)
         val account = AccountKdf.accountKeySeed(pass, salt)
 
-        // Same passphrase, same salt, same stretch — three unrelated secrets.
+        // Same passphrase, same salt, same stretch, three unrelated secrets.
         // If any pair matched, a compromise of one product's key would hand over
         // another's, which is the entire reason the contexts exist.
         assertFalse(auth.contentEquals(identity), "auth and identity keys collided")
@@ -86,8 +86,8 @@ class AccountKdfTest {
         assertEquals(32, account.size)
     }
 
-    // The tie between this KDF and FNID.encode — that a correctly derived seed
-    // also produces the same ADDRESS on every client — belongs here too, and
+    // The tie between this KDF and FNID.encode, that a correctly derived seed
+    // also produces the same ADDRESS on every client, belongs here too, and
     // cannot be written yet: the Kotlin FNID lives on the multi-homing branch
     // and is not on main. Add it when that lands, because a seed that derives
     // correctly and encodes differently still gives one account a different