fix: Signal adapter parity pass — integration gaps, clawdbot features, env var simplification

Integration gaps fixed (7 files missing Signal):
- cron/scheduler.py: Signal in platform_map (cron delivery was broken)
- agent/prompt_builder.py: PLATFORM_HINTS for Signal (agent knows it's on Signal)
- toolsets.py: hermes-signal toolset + added to hermes-gateway composite
- hermes_cli/status.py: Signal + Slack in platform status display
- tools/send_message_tool.py: Signal example in target description
- tools/cronjob_tools.py: Signal in delivery option docs + schema
- gateway/channel_directory.py: Signal in session-based channel discovery

Clawdbot parity features added to signal.py:
- Self-message filtering: prevents reply loops by checking sender != account
- SyncMessage filtering: ignores sync envelopes (sent transcripts, read receipts)
- Edit message support: reads dataMessage from editMessage envelope
- Mention rendering: replaces \uFFFC placeholders with @identifier text
- Jitter in SSE reconnection backoff (20% randomization, prevents thundering herd)

Env var simplification (7 → 4):
- Removed SIGNAL_DM_POLICY (DM auth follows standard platform pattern via
  SIGNAL_ALLOWED_USERS + DM pairing, same as Telegram/Discord)
- Removed SIGNAL_GROUP_POLICY (derived from SIGNAL_GROUP_ALLOWED_USERS:
  not set = disabled, set with IDs = allowlist, set with * = open)
- Removed SIGNAL_DEBUG (was setting root logger, removed entirely)
- Remaining: SIGNAL_HTTP_URL, SIGNAL_ACCOUNT (required),
  SIGNAL_ALLOWED_USERS, SIGNAL_GROUP_ALLOWED_USERS (optional)

Updated all docs (website, AGENTS.md, signal.md) to match.

gateway/config.py

@@ -399,8 +399,6 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
         config.platforms[Platform.SIGNAL].extra.update({
             "http_url": signal_url,
             "account": signal_account,
-            "dm_policy": os.getenv("SIGNAL_DM_POLICY", "pairing"),
-            "group_policy": os.getenv("SIGNAL_GROUP_POLICY", "disabled"),
             "ignore_stories": os.getenv("SIGNAL_IGNORE_STORIES", "true").lower() in ("true", "1", "yes"),
         })
         signal_home = os.getenv("SIGNAL_HOME_CHANNEL")