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/FileEnvelopeInteropTest.kt

@@ -10,9 +10,9 @@ import kotlin.test.assertFalse
 /**
  * Documents sent between the two apps.
  *
- * 🔴 **The envelope in [IOS_FILE] was produced by the iOS build**, not by this
+ * The envelope in [IOS_FILE] was produced by the iOS build, not by this
  * engine. Two implementations of one format that each only test against
- * themselves agree right up to the day somebody sends a real file — and the
+ * themselves agree right up to the day somebody sends a real file, and the
  * symptom is a message that arrives as "[unsupported attachment]" with no clue
  * why. This is the test that catches a drift in the field names before then.
  */
@@ -42,7 +42,7 @@ class FileEnvelopeInteropTest {
         assertEquals("application/pdf", ref.mime)
     }
 
-    /** 🔴 It must be recognised, not merely tolerated. */
+    /** It must be recognised, not merely tolerated. */
     @Test
     fun `a file is a known kind rather than needing a newer client`() {
         val content = Content.parse(iosFile)
@@ -77,7 +77,7 @@ class FileEnvelopeInteropTest {
     }
 
     /**
-     * 🔴 The compatibility rule, on this side too: never captionless, because
+     * The compatibility rule, on this side too: never captionless, because
      * a build that predates this kind shows only the caption.
      */
     @Test
@@ -102,7 +102,7 @@ class FileEnvelopeInteropTest {
         assertEquals("signed copy", Content.parse(Content.attachment(ref, "signed copy")).text)
     }
 
-    /** ⚠️ An older peer sends no `fn`/`cls`/`cmp`. It must parse, not throw. */
+    /** An older peer sends no `fn`/`cls`/`cmp`. It must parse, not throw. */
     @Test
     fun `an envelope without the new fields still parses`() {
         val older = """
@@ -117,7 +117,7 @@ class FileEnvelopeInteropTest {
     }
 
     /**
-     * 🔴 The additive property that makes any of this safe: a photo envelope
+     * The additive property that makes any of this safe: a photo envelope
      * from this engine still carries none of the file keys, so a peer that has
      * never heard of them reads it exactly as before.
      */
@@ -135,7 +135,7 @@ class FileEnvelopeInteropTest {
     }
 
     /**
-     * ⚠️ A filename from a peer is display text and a name, never a path.
+     * A filename from a peer is display text and a name, never a path.
      * Carried verbatim, and nothing here should ever join it onto a directory.
      */
     @Test