Write up what AI answer blocks would cost before building one

Google, Bing and Brave all put a generated answer above their results now,
and none of it is being read. Investigated against castle's Chrome; nothing
in src/ changed, because the cost turned out to be worth deciding on rather
than absorbing.

Two things make it expensive. Google's AI Overview exists only on the
classic layout — udm=14 is the "Web" tab, and having no overview is part of
what that tab is — so enabling it means giving back the clean page the
extractor was built around. And all three stream in after the results are
already parseable: Google settles ~2.3s in, Brave was still growing at 5s,
against an extractor that returns at ~0.5s. The wait is often wasted, since
the blocks are query-dependent — no Google overview for "how does postgres
MVCC vacuum work" after 15s, no Bing answer for "who founded anthropic".

Selectors, the traps behind them (#m-x-content briefly holds ~29k
characters of unrelated page; Google's "Show more" hides only the
disclaimer), the measured timings, three possible designs and their
tradeoffs are all in the doc, so this does not have to be re-measured if
the answer turns out to be yes.

1 files changed, 41 insertions, 0 deletions

diff --git a/castle-search/src/paths.ts b/castle-search/src/paths.ts
new file mode 100644
index 0000000..b008f4f
--- /dev/null
+++ b/castle-search/src/paths.ts
@@ -0,0 +1,41 @@
+/**
+ * pi's agent config directory, and the file pi-browser-harness stores its
+ * `/browser-target` choice in.
+ *
+ * This extension reads that file so a single `/browser-target` decision governs
+ * both packages. It never writes it — the harness owns that setting, and two
+ * writers of one file is a bug waiting to happen.
+ *
+ * pi exports `getAgentDir()`, but resolving it here would be a *runtime* import
+ * of the host package from an installed extension's own tree. Every other pi
+ * import in this codebase is `import type` and erased at build time, so the
+ * three lines are replicated instead (pi's dist/config.js: `PI_CODING_AGENT_DIR`,
+ * CONFIG_DIR_NAME `.pi`). Same reasoning as pi-browser-harness/src/profile/paths.ts.
+ */
+
+import { homedir } from "node:os";
+import { join } from "node:path";
+
+/** pi's env override for the agent dir (APP_NAME.toUpperCase() + "_CODING_AGENT_DIR"). */
+const ENV_AGENT_DIR = "PI_CODING_AGENT_DIR";
+
+/**
+ * Expand a leading `~` exactly as pi's expandTildePath does — `~` and `~/…`
+ * only, never `~\…`, on every platform. Being more lenient here would look in a
+ * different directory than pi resolves from the same variable.
+ */
+const expandTildePiCompatible = (path: string): string => {
+  if (path === "~") return homedir();
+  if (path.startsWith("~/")) return homedir() + path.slice(1);
+  return path;
+};
+
+/** pi's agent config directory — `$PI_CODING_AGENT_DIR` or `~/.pi/agent`. */
+export const agentDir = (): string => {
+  const fromEnv = process.env[ENV_AGENT_DIR];
+  if (fromEnv) return expandTildePiCompatible(fromEnv);
+  return join(homedir(), ".pi", "agent");
+};
+
+/** Where pi-browser-harness persists the `/browser-target` choice. */
+export const targetFilePath = (): string => join(agentDir(), "browser-target.json");