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/ControlFlagInteropTest.kt

@@ -8,9 +8,9 @@ import kotlin.test.assertTrue
 /**
  * The `ctl` flag, as all three engines actually write it.
  *
- * 🔴 **This engine wrote `"ctl":true` and the specification says `"ctl":1`.**
+ * This engine wrote `"ctl":true` and the specification says `"ctl":1`.
  * That was not a cosmetic disagreement. iOS decodes the field as a number
- * through a synthesised `Codable`, so a boolean did not fail to set the flag —
+ * through a synthesised `Codable`, so a boolean did not fail to set the flag,
  * it failed the whole envelope's decode and dropped the message onto the
  * plain-text path. An Android user turning on disappearing messages therefore
  * sent an iPhone a bubble containing raw JSON, and the timer was never applied
@@ -37,9 +37,9 @@ class ControlFlagInteropTest {
     }
 
     /**
-     * ⚠️ Both shapes are accepted on the way IN. `org.json`'s `optBoolean` does
+     * Both shapes are accepted on the way IN. `org.json`'s `optBoolean` does
      * not coerce a number, so reading only that missed every `"ctl":1` the
-     * other two engines send — and reading only the number would miss the
+     * other two engines send, and reading only the number would miss the
      * `true` this engine itself wrote, which is on phones now and cannot be
      * recalled.
      */
@@ -53,7 +53,7 @@ class ControlFlagInteropTest {
     }
 
     /**
-     * 🔴 The point of the flag: a control kind from a NEWER peer must be
+     * The point of the flag: a control kind from a NEWER peer must be
      * swallowed, not rendered. Without it, every control kind added after a
      * given release shows in that release as a bubble the person cannot act on.
      */