fix: scope getRoot to the comment tree so the shadow walk doesn't collect reddit chrome (closes task/getroot-comment-tree-scope)

Source: task/getroot-comment-tree-scope (plan/steward-linker-design) — getRoot's union querySelector resolved to <main> on new reddit, so the shadow-piercing walk over-collected reddit chrome.

- getRoot now tries shreddit-comment-tree, then .commentarea, then main/body sequentially (narrow-first) instead of one union querySelector, so the comment tree wins when present.
- collectInto takes an allowShadow flag: a narrow comment root pierces every open shadow host; a broad fallback root descends only into comment-component hosts (tagName SHREDDIT-COMMENT*), so chrome shadow roots (nav/sidebar/composer) are never walked.
- README + AGENTS shadow-DOM notes updated to describe the two-layer scoping.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENTS.md

@@ -24,16 +24,21 @@ is tracked in the **towl** work-item store under project **`reddit-spotify-linke
   phrases for albums) — node-testable via `extract.test.cjs`. Album links render
   italicised (`.rsl-album`).
   - **shreddit shadow DOM:** `collectTextNodes` is shadow-piercing. A TreeWalker can't
-    cross a shadow boundary, so it walks the comment-tree `root`'s light DOM AND
-    recursively descends into the **open** shadow roots of elements within that subtree.
-    New reddit's `shreddit-*` components may encapsulate comment bodies in a shadow root;
-    this reaches them whether reddit slots (light DOM) or encapsulates (shadow DOM), with
-    no need to know which. It is scoped to the comment-tree root (never the whole
-    document's shadow roots, which would pull in reddit chrome) and is a no-op on old
-    reddit (no shadow roots). Slotted light-DOM nodes are not double-collected (a shadow
-    walker sees the shadow tree's own nodes, not a `<slot>`'s assigned light nodes; a
-    Set de-dupes belt-and-braces). **Closed** shadow roots are unreachable from a content
-    script — a documented limitation.
+    cross a shadow boundary, so it walks the chosen `root`'s light DOM AND recursively
+    descends into the **open** shadow roots of elements within that subtree. New reddit's
+    `shreddit-*` components may encapsulate comment bodies in a shadow root; this reaches
+    them whether reddit slots (light DOM) or encapsulates (shadow DOM). Scope is two-layer:
+    `getRoot` tries the narrow comment containers first (`shreddit-comment-tree`, then old
+    reddit's `.commentarea`) before the broad `main`/`body`, so on a new-reddit comment page
+    the root is the comment tree and the walk stays within it; and when `getRoot` falls back
+    to a broad root (no comment tree present) the recursion descends only into comment-component
+    shadow hosts (`tagName` starting `SHREDDIT-COMMENT`), so reddit's chrome shadow roots (nav,
+    sidebar, composer) are never walked. (A single union `querySelector` could not do this — it
+    returns the first match in document order, and `<main>` is an ancestor of the comment tree,
+    so it would always win.) It is a no-op on old reddit (no shadow roots). Slotted light-DOM
+    nodes are not double-collected (a shadow walker sees the shadow tree's own nodes, not a
+    `<slot>`'s assigned light nodes; a Set de-dupes belt-and-braces). **Closed** shadow roots
+    are unreachable from a content script — a documented limitation.
 - `scripts/check.sh` — the local check gate.
 
 ## How work lands (IMPORTANT — this is NOT the towl repo)