build(channels): correct wordlist path for tool-side build invocations

shared/build-lib.sh is sourced from each tool's build.sh with
root_dir pointing at the tool directory, so paths to other shared/
artifacts have to be \$root_dir/../shared/<file>. The slug helper
incorrectly used \$root_dir/shared/build-words.txt, which only
resolves correctly when the helper is invoked from the repo root
(e.g. via the smoke test `root_dir=\$PWD sh -c …`). In a real
./build run, the helper found the wordlist missing and silently
fell back to the short-SHA — the previous beta cut shipped
"v0.0.17-beta · 2026-05-07 · 1dafc97" instead of the intended
slug.

Fix: align the path with the existing shared/favicon.svg pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

shared/build-lib.sh

@@ -176,7 +176,10 @@ _source_commit_ref() {
 _source_commit_slug() {
     _ref=$(_source_commit_ref)
     _full_sha=$(git -C "$root_dir" rev-parse "$_ref" 2>/dev/null || echo "0000000000000000")
-    _words_file="$root_dir/shared/build-words.txt"
+    # build-lib.sh is sourced from each tool's build.sh as
+    # "$root_dir/../shared/build-lib.sh"; the wordlist lives next to
+    # this file in the same shared/ directory.
+    _words_file="$root_dir/../shared/build-words.txt"
     if [ ! -f "$_words_file" ]; then
         # Fall back to plain short SHA if the wordlist is missing
         # (e.g. running from a checkout that predates this feature).