fix(parser): use real content digest for Shelley skip

Replace the additive/byte-length Shelley change signal with two
per-conversation signals so the sync skip cannot miss a rewrite:

- file_mtime stays the conversation's real updated_at timestamp. It must
  remain a true timestamp because ListSessionsModifiedBetween filters
  file_mtime <= now for PG/DuckDB push; a synthetic future value dropped
  a just-synced Shelley row from a same-second push until a later run.
- file_hash holds a content digest over the conversation's messages,
  computed identically by the meta skip query and the parse loop. It is
  length-framed, so it detects appends, in-place rewrites, and
  length-preserving same-second edits that a byte-length signal misses.

The bulk skip now compares the stored file_hash via GetFileHashByPath
alongside file_mtime. Computing the digest reads message payloads, which
the sub-second-timestamped siblings (Zed, Kiro) avoid; Shelley's
second-precision updated_at makes the read the price of not silently
skipping a rewrite. ShelleySourceMtime keeps a watcher-only change signal
(updated_at plus a sub-second digest term), compared for inequality and
never range-filtered.

Author: wesm

internal/db/sessions.go

@@ -1517,6 +1517,25 @@ func (db *DB) GetFileInfoByPath(
 	return s.Int64, m.Int64, true
 }
 
+// GetFileHashByPath returns the stored file_hash for the session
+// matching file_path, preferring the most recently modified row.
+// The bool is false when no row exists or the column is NULL. Used
+// by the Shelley skip to compare a per-conversation content
+// fingerprint alongside file_mtime.
+func (db *DB) GetFileHashByPath(path string) (hash string, ok bool) {
+	var h sql.NullString
+	err := db.getReader().QueryRow(
+		"SELECT file_hash FROM sessions"+
+			" WHERE file_path = ?"+
+			" ORDER BY file_mtime DESC LIMIT 1",
+		path,
+	).Scan(&h)
+	if err != nil {
+		return "", false
+	}
+	return h.String, h.Valid
+}
+
 // GetDataVersionByPath returns the minimum data_version for
 // sessions matching a file_path. Returns 0 when no session
 // exists for the path.