feat(mattermost): seed prior thread posts into first-turn context

cli-config.yaml.example

@@ -1401,6 +1401,14 @@ platform_toolsets:
 #   reactions: true                  # Show processing reactions (default: true)
 #   history_backfill: true           # Recover missed channel messages on mention (default: true)
 #   history_backfill_limit: 50       # Max messages to scan backwards (default: 50)
+#
+# Mattermost-specific settings (config.yaml top-level, not under platforms:):
+#
+# mattermost:
+#   require_mention: true            # Require @mention in server channels (default: true)
+#   reply_mode: "off"                # "thread" to nest replies under the root post
+#   free_response_channels: ""       # Channel IDs where no mention is needed
+#   thread_context: on               # Seed prior thread posts on first turn (on/off; default: on)
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Available toolsets (use these names in platform_toolsets or the toolsets list)

gateway/platforms/_shared.py

@@ -177,12 +177,23 @@ def send_error(message: Any) -> dict:
 
 # kind -> (applies-when predicate over (cfg, key), env encoder). "lower"/"json" bridge whenever the key
 # is present (YAML ``none`` still writes "none"); "str" skips null/blank, "csv" skips null and leaves
-# lists to the setter's comma join.
+# lists to the setter's comma join; "onoff" coerces a boolean-ish value to canonical "on"/"off".
+_ONOFF_OFF_TOKENS = {"off", "false", "0", "no", "none", ""}
+
+
+def _encode_onoff(value: Any) -> str:
+    if value is None:
+        return "on"
+    token = str(value).strip().lower()
+    return "off" if token in _ONOFF_OFF_TOKENS else "on"
+
+
 _YAML_KINDS: dict[str, tuple[Callable[[dict, str], bool], Callable[[Any], Any]]] = {
     "lower": (lambda cfg, key: key in cfg, lambda v: str(v).lower()),
     "str": (lambda cfg, key: cfg.get(key) not in (None, ""), str),
     "csv": (lambda cfg, key: cfg.get(key) is not None, lambda v: v),
     "json": (lambda cfg, key: key in cfg, json.dumps),
+    "onoff": (lambda cfg, key: key in cfg, _encode_onoff),
 }
 
 

hermes_cli/config_defaults.py

@@ -2824,6 +2824,10 @@ def _base_url(name, prompt_name=None):
     "MATTERMOST_FREE_RESPONSE_CHANNELS": _msg(
         "Comma-separated Mattermost channel IDs where bot responds without @mention",
         "Free-response channel IDs (comma-separated)", None),
+    "MATTERMOST_THREAD_CONTEXT": _msg(
+        "Seed prior thread posts into context on the first turn the bot is drawn into an "
+        "existing thread: on (default) or off.",
+        "Seed thread history on first turn (on/off)", None),
     "MATRIX_HOMESERVER": _msg("Matrix homeserver URL (e.g. https://matrix.example.org)",
         "Matrix homeserver URL", "https://matrix.org/ecosystem/servers/"),
     "MATRIX_ACCESS_TOKEN": _msg("Matrix access token (preferred over password login)",