fix(13-12): seed-phrase egress screen checks the real BIP39 wordlist

The shape-only heuristic ('any 12 consecutive lowercase 3-8-char words')
matched ordinary prose — including the node's own system prompt — and
blocked 100% of live cloud chat turns (found on dev3, the first real
Claude call through this screen; log: kind=bip39-word-run every turn).
Membership in the crate's own bip39 English wordlist (already a dep via
seed.rs) distinguishes prose from seed material: glue words break runs,
real seeds are nothing but members. Regression test pins the real system
prompt + a clean wire body to Allow; the 12-word genuine-seed case still
blocks. 13/13 egress tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/assistant/egress.rs

@@ -186,18 +186,23 @@ fn has_long_hex_run(body: &str, min_len: usize) -> bool {
     false
 }
 
-/// A run of exactly 12 or 24 lowercase-alphabetic "words" (BIP39 seed
-/// phrase length), each a plausible wordlist entry length (3-8 chars).
-/// Splits on ANY non-alphabetic character — not just whitespace — since
-/// `body` here is a raw JSON request string: a word sitting at the very
-/// end of a JSON string value is followed immediately by a closing `"`
-/// with no space at all, and splitting on whitespace alone would glue
-/// that word onto the rest of the JSON document as one giant non-matching
-/// token. Deliberately does not check against the real BIP39 wordlist
-/// (that would require bundling it here) — the LENGTH and SHAPE signature
-/// is what this heuristic exists to catch; a false positive fails closed
-/// (the request just doesn't leave the node), which is the correct
-/// direction to err in.
+/// A run of 12 or more consecutive REAL BIP39 wordlist entries — the shape
+/// of a seed phrase. Splits on ANY non-alphabetic character — not just
+/// whitespace — since `body` here is a raw JSON request string: a word
+/// sitting at the very end of a JSON string value is followed immediately
+/// by a closing `"` with no space at all, and splitting on whitespace
+/// alone would glue that word onto the rest of the JSON document as one
+/// giant non-matching token.
+///
+/// Checks membership in the crate's own `bip39` English wordlist (already
+/// a dependency via `seed.rs` — no bundling needed). The original
+/// shape-only heuristic ("any 12 consecutive lowercase 3-8-char words")
+/// matched ordinary English prose — including this node's OWN system
+/// prompt — and therefore blocked 100% of live cloud chat turns (found
+/// on-device on dev3, 2026-08-06, the first real Claude call through this
+/// screen). Function words that glue prose together ("the", "is", "of",
+/// "you") are not wordlist members, so real sentences break runs; real
+/// seed material is nothing but members.
 fn has_bip39_length_word_run(body: &str) -> bool {
     let words: Vec<&str> = body
         .split(|c: char| !c.is_ascii_alphabetic())
@@ -206,16 +211,19 @@ fn has_bip39_length_word_run(body: &str) -> bool {
     if words.len() < 12 {
         return false;
     }
-    let is_wordlike =
-        |w: &&str| (3..=8).contains(&w.len()) && w.chars().all(|c| c.is_ascii_lowercase());
-    for window_len in [24usize, 12usize] {
-        if words.len() < window_len {
-            continue;
-        }
-        for window in words.windows(window_len) {
-            if window.iter().all(is_wordlike) {
+    let wordlist = bip39::Language::English.word_list();
+    let mut run = 0usize;
+    for w in &words {
+        let lower = w.to_ascii_lowercase();
+        if w.chars().all(|c| c.is_ascii_lowercase())
+            && wordlist.binary_search(&lower.as_str()).is_ok()
+        {
+            run += 1;
+            if run >= 12 {
                 return true;
             }
+        } else {
+            run = 0;
         }
     }
     false
@@ -413,6 +421,34 @@ mod tests {
         );
     }
 
+    /// Regression (dev3 on-device, 2026-08-06): the node's OWN system
+    /// prompt — long, lowercase, node-authored English — must NOT read as
+    /// a seed phrase. The shape-only detector blocked 100% of live cloud
+    /// turns; wordlist membership is what distinguishes prose (function
+    /// words break runs) from seed material (nothing but members).
+    #[test]
+    fn real_system_prompt_is_not_a_seed_phrase() {
+        let registry = crate::assistant::tools::registry();
+        let all: std::collections::BTreeSet<_> =
+            crate::assistant::PermissionCategory::ALL.into_iter().collect();
+        let visible = registry.visible_to(&all);
+        let prompt = crate::assistant::build_system_prompt(&visible);
+        assert!(
+            !has_bip39_length_word_run(&prompt),
+            "the node's own system prompt must never trip the seed screen"
+        );
+        let body = json!({
+            "model": "claude-haiku-4-5",
+            "system": prompt,
+            "messages": [
+                {"role": "user", "content": "please restart filebrowser for me right now"},
+            ],
+        })
+        .to_string();
+        let ctx = ctx_for("please restart filebrowser for me right now", &[], &[]);
+        assert_eq!(screen_outbound(&body, &ctx), EgressVerdict::Allow);
+    }
+
     /// Behavior: an ecash-token-shaped string is blocked.
     #[test]
     fn ecash_token_shaped_string_is_blocked() {