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.

agent/prompt_builder.py

@@ -122,6 +122,15 @@ PLATFORM_HINTS = {
         "attachments, audio as file attachments. You can also include image URLs "
         "in markdown format ![alt](url) and they will be uploaded as attachments."
     ),
+    "signal": (
+        "You are on a text messaging communication platform, Signal. "
+        "Please do not use markdown as it does not render. "
+        "You can send media files natively: to deliver a file to the user, "
+        "include MEDIA:/absolute/path/to/file in your response. Images "
+        "(.png, .jpg, .webp) appear as photos, audio as attachments, and other "
+        "files arrive as downloadable documents. You can also include image "
+        "URLs in markdown format ![alt](url) and they will be sent as photos."
+    ),
     "cli": (
         "You are a CLI AI Agent. Try not to use markdown but simple text "
         "renderable inside a terminal."