fix(compaction): failures carry the real error, with endpoint credentials redacted

The pager rendered "Compaction failed." with the error dropped, and the
shell's unsuppressed auto-compaction failure sent an empty error. Both
now carry the decoded first line of the failure (retry guidance plus the
detail), decoded from the ACP data rather than its JSON-escaped Display,
propagated unchanged through the compact extension, capped for a banner.

Endpoint credentials never reach the banner: the sampler redacts the
parsed endpoint (userinfo, sensitive query values, fragment via the
repository sanitizer) in every 401 message and the appended request URL,
and the shell redacts every URL in the error text before line selection,
including URLs whose password holds whitespace or that follow a
multi-byte delimiter.

Ported from upstream 1.0.13 (grok-build dispatch/prompt.rs,
session/compaction.rs), extended with the redaction.

Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4

crates/codegen/kigi-shell/src/sampling/error.rs

@@ -171,6 +171,70 @@ pub fn error_detail_from_data(data: &serde_json::Value) -> Option<String> {
         .map(str::to_owned)
 }
 
+/// The human-readable text of an ACP error: its `data` detail when present (Display would JSON-escape it), else the message.
+pub fn acp_error_message(err: &acp::Error) -> String {
+    err.data
+        .as_ref()
+        .and_then(error_detail_from_data)
+        .unwrap_or_else(|| err.message.clone())
+}
+
+/// First line of a compaction error without internal `compact failed:` prefixes, capped for a banner; diagnostic lines (request URLs) are dropped.
+pub fn user_facing_compact_error(raw: &str) -> String {
+    const INTERNAL_PREFIX: &str = "compact failed:";
+    const MAX_CHARS: usize = 200;
+    // Redact before line selection: a line break inside a URL must not hide its `@`.
+    let raw = redact_urls_in_text(raw);
+    let mut rest = raw.trim();
+    while rest
+        .get(..INTERNAL_PREFIX.len())
+        .is_some_and(|head| head.eq_ignore_ascii_case(INTERNAL_PREFIX))
+    {
+        rest = rest[INTERNAL_PREFIX.len()..].trim_start();
+    }
+    rest.lines()
+        .next()
+        .unwrap_or_default()
+        .trim()
+        .chars()
+        .take(MAX_CHARS)
+        .collect()
+}
+
+/// Every canonical `scheme://` URL in `text` loses userinfo, sensitive query values and fragment; one that fails to parse is not echoed.
+fn redact_urls_in_text(text: &str) -> String {
+    let mut out = String::with_capacity(text.len());
+    let mut rest = text;
+    while let Some(sep) = rest.find("://") {
+        // The delimiter before the scheme may be multi-byte (`“`, `：`): step past its UTF-8 length.
+        let scheme_start = rest[..sep]
+            .char_indices()
+            .rev()
+            .find(|(_, c)| !(c.is_ascii_alphanumeric() || matches!(c, '+' | '-' | '.')))
+            .map_or(0, |(at, c)| at + c.len_utf8());
+        out.push_str(&rest[..scheme_start]);
+        // The authority may hold whitespace (a configured password); the token ends at whitespace after it.
+        let authority = &rest[sep + 3..];
+        let authority_end = authority.find(['/', '?', '#']).unwrap_or(authority.len());
+        let token_end = authority[authority_end..]
+            .find(char::is_whitespace)
+            .map_or(rest.len(), |at| sep + 3 + authority_end + at);
+        let token = &rest[scheme_start..token_end];
+        let url_text = token.trim_end_matches([')', ']', '}', ',', '.', ';', '\'', '"', '>']);
+        match url::Url::parse(url_text) {
+            Ok(mut url) => {
+                kigi_secrets::redact_url(&mut url);
+                out.push_str(url.as_str());
+            }
+            Err(_) => out.push_str("<unparseable url>"),
+        }
+        out.push_str(&token[url_text.len()..]);
+        rest = &rest[token_end..];
+    }
+    out.push_str(rest);
+    out
+}
+
 pub fn http_status_from_error(err: &acp::Error) -> Option<u16> {
     err.data
         .as_ref()?
@@ -254,6 +318,77 @@ pub fn prompt_complete_fields(
 
 #[cfg(test)]
 mod tests {
+    #[test]
+    fn compact_error_text_keeps_the_first_decoded_line_only() {
+        let err = super::acp::Error::internal_error().data(
+            "compact failed: API error (status 500): overloaded\n\nRequest URL: http://u:TEST_PASSWORD@127.0.0.1/v1",
+        );
+        let detail = super::user_facing_compact_error(&super::acp_error_message(&err));
+        assert_eq!(detail, "API error (status 500): overloaded");
+        assert!(!detail.contains("TEST_PASSWORD"));
+        assert_eq!(
+            super::user_facing_compact_error(&"x".repeat(500))
+                .chars()
+                .count(),
+            200
+        );
+        assert_eq!(
+            super::acp_error_message(&super::acp::Error::internal_error()),
+            "Internal error"
+        );
+    }
+
+    /// A sampler 401 names the endpoint on the retained line; its userinfo must not survive.
+    #[test]
+    fn compact_error_text_redacts_url_credentials_on_the_first_line() {
+        for password in [
+            "TEST_PASSWORD",
+            "TEST PASSWORD",
+            "TEST%20PASSWORD",
+            "TEST_PASSWORD\n",
+            "TEST_PASSWORD\r\n",
+        ] {
+            let raw = format!(
+                "compact failed: Unauthorized (401) from http://u:{password}@127.0.0.1:33263/401/v1/chat/completions: bad auth"
+            );
+            assert_eq!(
+                super::user_facing_compact_error(&raw),
+                "Unauthorized (401) from http://127.0.0.1:33263/401/v1/chat/completions: bad auth",
+                "{password:?}"
+            );
+        }
+        assert_eq!(
+            super::redact_urls_in_text("see https://a:b@h.example/x and http://h2/"),
+            "see https://h.example/x and http://h2/"
+        );
+    }
+
+    /// A credential carried as a query value survives no failure shape: 401 message or transport error.
+    #[test]
+    fn compact_error_text_redacts_query_credentials_in_every_failure_shape() {
+        let unauthorized = "compact failed: Unauthorized (401) from http://127.0.0.1:1/?access_token=TEST_QUERY_TOKEN&route=/v1/chat/completions: bad auth";
+        let transport = "compact failed: request error: error sending request for url (http://127.0.0.1:1/?access_token=TEST_QUERY_TOKEN&route=/500/v1/chat/completions)";
+        for raw in [unauthorized, transport] {
+            let detail = super::user_facing_compact_error(raw);
+            assert!(!detail.contains("TEST_QUERY_TOKEN"), "{detail}");
+            assert!(detail.contains("route="), "{detail}");
+        }
+        assert!(super::user_facing_compact_error("see http://:://").contains("<unparseable url>"));
+    }
+
+    /// A multi-byte delimiter right before the URL must not break the scan.
+    #[test]
+    fn compact_error_text_handles_unicode_before_a_url() {
+        for raw in [
+            "compact failed: Service unavailable. See “https://u:TEST_PASSWORD@status.example/”.",
+            "compact failed: 请参阅：https://u:TEST_PASSWORD@status.example/",
+        ] {
+            let detail = super::user_facing_compact_error(raw);
+            assert!(!detail.contains("TEST_PASSWORD"), "{detail}");
+            assert!(detail.contains("https://status.example/"), "{detail}");
+        }
+    }
+
     use super::*;
     use reqwest::StatusCode;