feat(streaming): per-platform streaming defaults (Telegram on, Discord off) + dashboard toggles (#37303)

Streaming quality differs sharply by platform: Telegram has native animated
draft streaming (sendMessageDraft) which is smooth, while Discord/Slack only
have edit-based streaming (repeated editMessage) which visibly flickers. Ship
defaults that match reality instead of one global flag.

- hermes_cli/config.py: DEFAULT_CONFIG display.platforms now ships
  telegram.streaming=true and discord.streaming=false (was empty {}). These
  are gap-fillers — config deep-merge has user values win, so anyone who
  explicitly sets discord.streaming=true keeps it. The global
  streaming.enabled master switch still gates everything; these per-platform
  flags only take effect once streaming is on.
- Dashboard exposure comes for free: the web settings schema is generated
  from DEFAULT_CONFIG, so display.platforms.telegram.streaming and
  .discord.streaming now surface as editable boolean toggles in the UI with
  no frontend change. (Previously the per-platform tree was {} and invisible.)
- tests: pin the defaults, the resolver outcome (telegram on / discord off /
  unlisted platforms follow global), user-override-wins, and dashboard schema
  exposure.

No _config_version bump: deep-merge fills the gap for existing installs; no
value migration needed.

hermes_cli/config.py

@@ -1345,7 +1345,27 @@ DEFAULT_CONFIG = {
         # responses and content messages are never touched.  Default 0
         # (disabled) preserves prior behavior.
         "ephemeral_system_ttl": 0,
-        "platforms": {},  # Per-platform display overrides: {"telegram": {"tool_progress": "all"}, "slack": {"tool_progress": "off"}}
+        # Per-platform display/streaming overrides. Each key is a gateway
+        # platform ("telegram", "discord", "slack", …) mapping to a dict of
+        # display settings that override the global value for that platform
+        # only. A setting left unset here falls through to the global default.
+        #
+        # Shipped defaults encode the streaming experience that works best
+        # per platform:
+        #   - Telegram has native animated draft streaming (sendMessageDraft),
+        #     which is smooth, so streaming is on by default there.
+        #   - Discord/Slack/etc. only have edit-based streaming (repeated
+        #     editMessage), which flickers and is noticeably jankier, so
+        #     streaming is off by default there.
+        # These are gap-fillers: a user who explicitly sets, e.g.,
+        # display.platforms.discord.streaming: true keeps their value
+        # (config deep-merge has user values win over defaults). The global
+        # streaming.enabled master switch still gates everything — these
+        # per-platform flags only take effect once streaming is enabled.
+        "platforms": {
+            "telegram": {"streaming": True},
+            "discord": {"streaming": False},
+        },
         # Gateway runtime-metadata footer appended to the FINAL message of a turn
         # (disabled by default to keep replies minimal). When enabled, renders
         # e.g. `model · 68% · ~/projects/hermes`. Per-platform overrides go under