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/BridgeOfferTest.kt

@@ -10,9 +10,9 @@ import kotlin.test.assertTrue
 /**
  * Handing a contact the bridge you use (§6.19).
  *
- * 🔴 **The security property under test is that this is an OFFER.** A bridge
+ * The security property under test is that this is an OFFER. A bridge
  * line routes every relay connection through a machine somebody else chose. A
- * contact — or whoever has taken their account — could not read the traffic,
+ * contact, or whoever has taken their account, could not read the traffic,
  * because the ratchet does not care what carries it, but they would learn who
  * is online and when, and could cut them off. So nothing in the engine or the
  * transport may apply one; only the person it was sent to can.
@@ -33,8 +33,8 @@ class BridgeOfferTest {
     }
 
     /**
-     * 🔴 **`1`, not `true`.** iOS decodes `ctl` as a number, and a boolean does
-     * not merely fail to set the flag — it fails the whole envelope's decode
+     * `1`, not `true`. iOS decodes `ctl` as a number, and a boolean does
+     * not merely fail to set the flag, it fails the whole envelope's decode
      * and falls through to the plain-text path, which is how an Android user
      * once sent an iPhone a bubble of raw JSON. Asserted on the wire rather
      * than through the parser, because the parser here would accept both.
@@ -47,7 +47,7 @@ class BridgeOfferTest {
     }
 
     /**
-     * 🔴 **What an older build does with this.** A build that has never heard
+     * What an older build does with this. A build that has never heard
      * of this kind must swallow it, not draw it. `ctl` is what makes that true,
      * and membership of CONTROL is what routes it to a handler here.
      */
@@ -59,7 +59,7 @@ class BridgeOfferTest {
         assertTrue(Content.Kind.BRIDGE in Content.Kind.CONTROL)
     }
 
-    /** ⚠️ The line crosses whole. Rebuilding it from parsed fields at the far
+    /** The line crosses whole. Rebuilding it from parsed fields at the far
      *  end would lose the `cert=` that makes it usable and the fingerprint
      *  somebody may want to check against another source. */
     @Test
@@ -68,7 +68,7 @@ class BridgeOfferTest {
         assertEquals(awkward, Content.bridgeLine(Content.parse(Content.bridgeOffer(awkward))))
     }
 
-    /** ⚠️ Asked of the wrong kind, it says nothing rather than guessing. */
+    /** Asked of the wrong kind, it says nothing rather than guessing. */
     @Test
     fun `another kind carries no bridge line`() {
         assertNull(Content.bridgeLine(Content.parse(Content.disappearing(60))))
@@ -76,7 +76,7 @@ class BridgeOfferTest {
     }
 
     /**
-     * ⚠️ An empty or whitespace line is not an offer. Storing one would put an
+     * An empty or whitespace line is not an offer. Storing one would put an
      * un-actionable row on the settings screen with somebody's name against it.
      */
     @Test
@@ -85,10 +85,10 @@ class BridgeOfferTest {
     }
 
     /**
-     * 🔴 **The engine validates nothing, deliberately.** Whether a line is
+     * The engine validates nothing, deliberately. Whether a line is
      * usable is asked on the screen where a person decides, so a bad one is
      * reported to them with a reason. Rejecting it here would drop it in
-     * silence — the exact failure the settings screen was built to end.
+     * silence, the exact failure the settings screen was built to end.
      */
     @Test
     fun `a malformed line still arrives, to be judged where it can be explained`() {