fix(slack): expose thread_ts in sender_json so agents reply in thread

[dogzzdogzz]

Problem

When an agent calls chat.postMessage directly via the Slack API, it needs to pass thread_ts to keep replies in the correct thread. Previously, SenderContext serialized the thread identifier as \"thread_id\" — a generic platform-agnostic name that agents could not reliably map to Slack's thread_ts parameter. As a result, messages were posted to channel root instead of the thread.

This is a Slack-only issue. Discord threads are separate channels, so channel_id already identifies the thread — no separate thread_ts concept exists.

Discussion: https://discord.com/channels/1491295327620169908/1494739741600387204/1494944277694775457

Solution

Change AdapterRouter::handle_message to accept a pre-serialized sender_json: &str so each platform adapter controls its own serialization format:

• slack.rs: renames \"thread_id\" → \"thread_ts\" before passing to handle_message
• discord.rs: serializes as-is (no change in behavior)
• adapter.rs: removes internal serde_json::to_string(sender) call

Agent now receives:
```json
{
"channel": "slack",
"channel_id": "C99999",
"thread_ts": "1234567890.123456"
}
```

No shared layer is polluted with Slack-specific logic.

Files Changed

• src/adapter.rs — signature change only
• src/slack.rs — custom serialization with thread_ts key
• src/discord.rs — standard serialization, no behavior change

Test Plan

• cargo clippy -- -D warnings passes
• Slack: agent replies with thread_ts → message stays in thread
• Discord: behavior unchanged

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