feat(text): 'get text --pierce' reads through CLOSED shadow DOM (#30)

Some injected UI (browser-extension debug panels, web components) renders into a
CLOSED shadow root that eval/innerText cannot read. --pierce walks the CDP DOM
tree (DOM.getDocument depth:-1 pierce:true), which includes closed shadow roots
and child documents, and collects text nodes (skipping script/style/etc).

Review-safe: rides the per-tab debugger session already attached, no new Chrome
permission and no ab-connect/extension change — so it works in extension-relay
mode without touching the published extension. Verified live: a closed-shadow
panel that main-world eval reports HIDDEN is read in full via --pierce.

First slice of #30 (read extension/injected-panel content). Deeper extension
introspection (background SW / chrome.storage) stays a launch-mode / raw-CDP
concern, deliberately NOT done by expanding ab-connect's debugger powers.

cli/src/output.rs

@@ -1961,6 +1961,7 @@ Retrieves various types of information from elements or the page.
 Subcommands:
   text [selector]            Element text; no selector = WHOLE PAGE, all frames
   text --main                Main-content text only (skip nav/header/sidebar)
+  text --pierce              Read through CLOSED shadow DOM (injected panels)
   html <selector>            Get inner HTML of element
   value <selector>           Get value of input element
   attr <selector> <name>     Get attribute value