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/DisappearingTest.kt

@@ -9,7 +9,7 @@ import kotlin.test.assertTrue
 /**
  * Disappearing messages (§6.11), at the vault.
  *
- * 🔴 **Expiry runs on read, and that is the point.** A sweep on a timer leaves
+ * Expiry runs on read, and that is the point. A sweep on a timer leaves
  * expired messages on disk between runs, and a "disappearing" message still
  * sitting in the vault is a claim rather than a feature. Reading is the one
  * path every caller shares.
@@ -51,7 +51,7 @@ class DisappearingTest {
         assertEquals("fresh", kept.getJSONObject(0).optString("text"))
     }
 
-    /** ⚠️ Gone from disk, not merely filtered out of the answer. */
+    /** Gone from disk, not merely filtered out of the answer. */
     @Test
     fun `expiry is a deletion, not a filter`() {
         val v = vault()
@@ -66,7 +66,7 @@ class DisappearingTest {
     }
 
     /**
-     * ⚠️ A message with no timestamp predates the field. An unknown age is not
+     * A message with no timestamp predates the field. An unknown age is not
      * evidence of an old one, and deleting somebody's history on a guess cannot
      * be undone.
      */
@@ -98,7 +98,7 @@ class DisappearingTest {
         assertEquals(0, v.disappearSeconds("t"))
     }
 
-    /** Exactly at the boundary the message survives — off-by-one deletes data. */
+    /** Exactly at the boundary the message survives, off-by-one deletes data. */
     @Test
     fun `a message exactly at the cutoff survives`() {
         val v = vault()