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/ContactCardTest.kt

@@ -8,13 +8,13 @@ import kotlin.test.assertTrue
 /**
  * Parsing an address, and refusing everything that is nearly one.
  *
- * 🔴 **This runs on input nobody authenticated.** An address arrives typed, or
- * pasted, or scanned from a code somebody else printed — and, worst of all,
+ * This runs on input nobody authenticated. An address arrives typed, or
+ * pasted, or scanned from a code somebody else printed, and, worst of all,
  * from inside a sealed envelope written by a stranger on another relay. So the
  * refusals matter more than the successes: accepting `fnid@https://x/y` would
  * let a message point this client at a path on a host of the sender's choosing.
  *
- * ⚠️ The cases here deliberately mirror the Swift suite one for one. Two
+ * The cases here deliberately mirror the Swift suite one for one. Two
  * implementations that disagree about which strings are addresses open two
  * threads for one person on one side and none on the other, and nothing about
  * either engine's own tests would show it.
@@ -45,7 +45,7 @@ class ContactCardTest {
     }
 
     /**
-     * ⚠️ Kept only so an older card still parses. Nothing reads it and nothing
+     * Kept only so an older card still parses. Nothing reads it and nothing
      * should start to: sealing uses the recipient DEVICE's key, which the
      * remote directory returns, and the federated send carries no delivery key
      * at all.
@@ -62,8 +62,8 @@ class ContactCardTest {
     }
 
     /**
-     * 🔴 A relay is a hostname. Accepting a URL here would let a card — or a
-     * message from a stranger — point this client at a path on somebody else's
+     * A relay is a hostname. Accepting a URL here would let a card, or a
+     * message from a stranger, point this client at a path on somebody else's
      * host.
      */
     @Test
@@ -83,7 +83,7 @@ class ContactCardTest {
     }
 
     /**
-     * ⚠️ A second `@` means this is not one address. Splitting on the first
+     * A second `@` means this is not one address. Splitting on the first
      * would silently address a different host than the string names.
      */
     @Test
@@ -92,7 +92,7 @@ class ContactCardTest {
     }
 
     /**
-     * 🔴 Two spellings of one address would open two threads for one person, so
+     * Two spellings of one address would open two threads for one person, so
      * everything that files a message goes through the same normaliser.
      */
     @Test