fix(unified): port reaction_add handler to unified build

[chaodu-agent]

Problem

The unified binary (Dockerfile.unified / pre-beta-* images) was missing the reaction_add EventHandler method. Emoji reaction mappings configured via [reactions.mapping] silently did nothing.

Root Cause

When the workspace was restructured (PR #1146), reaction_add was never ported from src/discord.rs to crates/openab-core/src/discord.rs.

Fix

Ported the full reaction_add handler (~247 lines) from the standard build. Removed ctl_registry references (IPC module not yet in unified — tracked separately).

Testing

• cargo check --features unified ✅
• cargo clippy --features unified -- -D warnings ✅

Fixes #1176

[chaodu-agent]

LGTM ✅ — Clean port of the reaction_add handler to the unified build.

What This PR Does

The unified binary (Dockerfile.unified / pre-beta-* images) silently dropped emoji-to-text reaction mappings because reaction_add was never ported during the workspace restructure in PR #1146. This PR restores that functionality.

How It Works

Ports the full reaction_add EventHandler (~247 lines) from the standard build into crates/openab-core/src/discord.rs. The handler:

1. Ignores bot self-reactions (feedback loop prevention)
2. Extracts unicode emoji → looks up [reactions.mapping] config
3. Applies the same bot/user/channel gating as message() (AllowBots, AllowUsers, allowlists, multibot detection)
4. Resolves thread context and dispatches through the existing Dispatcher
5. Intentionally omits ctl_registry references (IPC not yet in unified — tracked separately)

Findings

#	Severity	Finding	Location
1	🟢	Faithful port with correct gating parity	discord.rs:903-1152
2	🟢	Defense-in-depth bot re-check after to_user() fetch	discord.rs:1017-1028
3	🟢	Multibot routing uses message_author_id correctly	discord.rs:1094-1098

Finding Details

🟢 F1: Faithful port with correct gating parity

The handler replicates all access-control gates from message(): bot gating (AllowBots enum), user allowlist (is_denied_user), channel/thread detection, AllowUsers mode gating, and multibot thread routing. No logic drift.

🟢 F2: Defense-in-depth bot re-check

After fetching user info via to_user(), the handler re-applies bot gating if the API reveals the reactor is a bot but reaction.member was None. Edge case handled correctly.

🟢 F3: Multibot routing

In MultibotMentions mode, the handler correctly uses reaction.message_author_id to determine which bot should respond — only processing reactions on messages authored by this bot.

Baseline Check

• PR opened: 2026-06-23
• Main already has: reactions.mapping config parsing, ReactionsConfig struct with mapping: HashMap<String, String>, but no reaction_add handler in the unified crate
• Net-new value: Restores the entire event handler that makes [reactions.mapping] actually work in unified builds

What's Good (🟢)

• Exact parity with standard build gating logic — no shortcuts taken
• Clean removal of ctl_registry with explicit tracking note in PR body
• Early-exit pattern (fast returns before expensive API calls)
• CI cargo check and clippy pass