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>

scripts/secret-sweep.sh

@@ -3,7 +3,7 @@
 # Refuse to publish anything that looks like a credential.
 #
 # This repository is PUBLIC, and a public git history is
-# not something you can take back — a leaked key is leaked the moment the push
+# not something you can take back, a leaked key is leaked the moment the push
 # lands, regardless of what the next commit does. So this runs as a gate before
 # a push, not as a report afterwards.
 #
@@ -24,12 +24,12 @@ cd "$ROOT"
 # find, and a repo with a `Scripts/` rather than a `scripts/` did exactly that.
 # `git ls-files .` and not plain `git ls-files`: this subtree may be untracked
 # in the repo that contains it, and an unscoped call there returns the WHOLE
-# repository. An empty result means untracked, not empty — fall back to find,
+# repository. An empty result means untracked, not empty, fall back to find,
 # or the sweep silently passes by looking at nothing, which is worse than not
 # running it at all.
-# 🔴 **`--others` matters as much as `--cached`.** Plain `git ls-files` lists
-# only TRACKED files, so a brand-new one — which is exactly what a dropped
-# credential looks like — was invisible to this sweep while `git add -A` staged
+# `--others` matters as much as `--cached`. Plain `git ls-files` lists
+# only TRACKED files, so a brand-new one, which is exactly what a dropped
+# credential looks like, was invisible to this sweep while `git add -A` staged
 # it happily. A planted `sk_live_…` in a new file passed the gate. This set is
 # the one `git add -A` would stage: tracked, plus untracked and not ignored.
 FILES=$(git ls-files --cached --others --exclude-standard . 2>/dev/null || true)