feat(whatsapp): propagate sender identity to agent context

[f-liva]

Summary

Fixes #597 — WhatsApp sender identity metadata was silently discarded in the message pipeline, causing agents to treat all users as their owner.

• Add metadata field to MessageRequest — accepts sender info from channel gateways (WhatsApp, etc.)
• Create SenderContext struct — typed representation of channel + sender_id + sender_name
• Propagate through kernel — send_message_with_handle_and_blocks() now accepts optional SenderContext
• Inject into system prompt — agents see sender name and platform ID in their "Channel Awareness" section
• Activate allowed_users filtering — open-by-default (empty list = accept all), configurable per-channel
• Make is_allowed() public — remove #[allow(dead_code)], available for external callers

Files changed

File	Change
openfang-api/src/types.rs	Add metadata: Option<HashMap<String, Value>> to MessageRequest
openfang-api/src/routes.rs	Parse metadata → SenderContext, add allowed_users check
openfang-types/src/message.rs	New SenderContext struct
openfang-kernel/src/kernel.rs	Thread SenderContext through execute pipeline
openfang-runtime/src/prompt_builder.rs	Render sender identity in system prompt
openfang-channels/src/whatsapp.rs	Make is_allowed() public, open-by-default

Behavior

Before: Agent receives WhatsApp message with no sender context → treats everyone as owner
After: Agent sees sender identity in system prompt → can apply privacy rules and distinguish users

# config.toml — optional, open by default
[channels.whatsapp]
allowed_users = ["+391234567890"]  # empty or omitted = accept all

Test plan

• Send WhatsApp message from owner's number → agent receives sender context, identifies as owner
• Send WhatsApp message from unknown number (no allowed_users set) → message accepted, agent sees sender as non-owner
• Set allowed_users in config → messages from unlisted numbers return 403
• Telegram and other channels unaffected (no metadata = no sender context injected)
• cargo build --lib compiles cleanly

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