fix(gateway): canonicalize WhatsApp identity in session keys

Hermes' WhatsApp bridge routinely surfaces the same person under either
a phone-format JID (60123456789@s.whatsapp.net) or a LID (…@lid),
and may flip between the two for a single human within the same
conversation. Before this change, build_session_key used the raw
identifier verbatim, so the bridge reshuffling an alias form produced
two distinct session keys for the same person — in two places:

  1. DM chat_id — a user's DM sessions split in half, transcripts and
     per-sender state diverge.
  2. Group participant_id (with group_sessions_per_user enabled) — a
     member's per-user session inside a group splits in half for the
     same reason.

Add a canonicalizer that walks the bridge's lid-mapping-*.json files
and picks the shortest/numeric-preferred alias as the stable identity.
build_session_key now routes both the DM chat_id and the group
participant_id through this helper when the platform is WhatsApp.
All other platforms and chat types are untouched.

Expose canonical_whatsapp_identifier and normalize_whatsapp_identifier
as public helpers. Plugins that need per-sender behaviour (role-based
routing, per-contact authorization, policy gating) need the same
identity resolution Hermes uses internally; without a public helper,
each plugin would have to re-implement the walker against the bridge's
internal on-disk format. Keeping this alongside build_session_key
makes it authoritative and one refactor away if the bridge ever
changes shape.

_expand_whatsapp_aliases stays private — it's an implementation detail
of how the mapping files are walked, not a contract callers should
depend on.

gateway/session.py

@@ -60,6 +60,7 @@ from .config import (
     SessionResetPolicy,  # noqa: F401 — re-exported via gateway/__init__.py
     HomeChannel,
 )
+from hermes_constants import get_hermes_home
 
 
 @dataclass
@@ -518,15 +519,24 @@ def build_session_key(
     """
     platform = source.platform.value
     if source.chat_type == "dm":
-        if source.chat_id:
+        dm_chat_id = source.chat_id
+        if source.platform == Platform.WHATSAPP:
+            dm_chat_id = canonical_whatsapp_identifier(source.chat_id)
+
+        if dm_chat_id:
             if source.thread_id:
-                return f"agent:main:{platform}:dm:{source.chat_id}:{source.thread_id}"
-            return f"agent:main:{platform}:dm:{source.chat_id}"
+                return f"agent:main:{platform}:dm:{dm_chat_id}:{source.thread_id}"
+            return f"agent:main:{platform}:dm:{dm_chat_id}"
         if source.thread_id:
             return f"agent:main:{platform}:dm:{source.thread_id}"
         return f"agent:main:{platform}:dm"
 
     participant_id = source.user_id_alt or source.user_id
+    if participant_id and source.platform == Platform.WHATSAPP:
+        # Same JID/LID-flip bug as the DM case: without canonicalisation, a
+        # single group member gets two isolated per-user sessions when the
+        # bridge reshuffles alias forms.
+        participant_id = canonical_whatsapp_identifier(str(participant_id)) or participant_id
     key_parts = ["agent:main", platform, source.chat_type]
 
     if source.chat_id:
@@ -547,6 +557,95 @@ def build_session_key(
     return ":".join(key_parts)
 
 
+def normalize_whatsapp_identifier(value: str) -> str:
+    """Strip WhatsApp JID/LID syntax down to its stable numeric identifier.
+
+    Accepts any of the identifier shapes the WhatsApp bridge may emit:
+    ``"60123456789@s.whatsapp.net"``, ``"60123456789:47@s.whatsapp.net"``,
+    ``"60123456789@lid"``, or a bare ``"+60123456789"`` / ``"60123456789"``.
+    Returns just the numeric identifier (``"60123456789"``) suitable for
+    equality comparisons.
+
+    Useful for plugins that want to match sender IDs against
+    user-supplied config (phone numbers in ``config.yaml``) without
+    worrying about which variant the bridge happens to deliver.
+    """
+    return (
+        str(value or "")
+        .strip()
+        .replace("+", "", 1)
+        .split(":", 1)[0]
+        .split("@", 1)[0]
+    )
+
+
+def _expand_whatsapp_aliases(identifier: str) -> set[str]:
+    """Resolve WhatsApp phone/LID aliases using bridge session mapping files."""
+    normalized = normalize_whatsapp_identifier(identifier)
+    if not normalized:
+        return set()
+
+    session_dir = get_hermes_home() / "whatsapp" / "session"
+    resolved: set[str] = set()
+    queue = [normalized]
+
+    while queue:
+        current = queue.pop(0)
+        if not current or current in resolved:
+            continue
+
+        resolved.add(current)
+        for suffix in ("", "_reverse"):
+            mapping_path = session_dir / f"lid-mapping-{current}{suffix}.json"
+            if not mapping_path.exists():
+                continue
+            try:
+                mapped = normalize_whatsapp_identifier(
+                    json.loads(mapping_path.read_text(encoding="utf-8"))
+                )
+            except Exception:
+                continue
+            if mapped and mapped not in resolved:
+                queue.append(mapped)
+
+    return resolved
+
+
+def canonical_whatsapp_identifier(identifier: str) -> str:
+    """Return a stable WhatsApp sender identity across phone-JID/LID variants.
+
+    WhatsApp may surface the same person under either a phone-format JID
+    (``60123456789@s.whatsapp.net``) or a LID (``1234567890@lid``). This
+    applies to a DM ``chat_id`` *and* to the ``participant_id`` of a
+    member inside a group chat — both represent a user identity, and the
+    bridge may flip between the two for the same human.
+
+    This helper reads the bridge's ``whatsapp/session/lid-mapping-*.json``
+    files, walks the mapping transitively, and picks the shortest
+    (numeric-preferred) alias as the canonical identity. :func:`build_session_key`
+    uses this for both WhatsApp DM chat_ids and WhatsApp group participant_ids,
+    so callers get the same session-key identity Hermes itself uses.
+
+    Plugins that need per-sender behaviour (role-based routing,
+    authorization, per-contact policy) should use this so their
+    bookkeeping lines up with Hermes' session bookkeeping even when
+    the bridge reshuffles aliases.
+
+    Returns an empty string if ``identifier`` normalizes to empty. If no
+    mapping files exist yet (fresh bridge install), returns the
+    normalized input unchanged.
+    """
+    normalized = normalize_whatsapp_identifier(identifier)
+    if not normalized:
+        return ""
+
+    # _expand_whatsapp_aliases always includes `normalized` itself in the
+    # returned set, so the min() below degrades gracefully to `normalized`
+    # when no lid-mapping files are present.
+    aliases = _expand_whatsapp_aliases(normalized)
+    return min(aliases, key=lambda candidate: (len(candidate), candidate))
+
+
 class SessionStore:
     """
     Manages session storage and retrieval.