fix: role-mention only matches bot's own roles (with guild_create cache)

[chengli]

Summary

• Fix role-mention detection to only match roles assigned to the receiving bot, preventing cross-triggers in multi-bot guilds
• Cache the bot's own role IDs from guild_create events (not ready) in a RwLock<HashSet> — zero per-message API calls
• Early-exit when msg.mention_roles is empty (skip cache read entirely)

Closes #245

Details

Bug: msg.mention_roles.iter().any(|r| msg.content.contains(...)) triggers on any role mentioned in the message, not just the bot's own roles. In a guild with bots A (role X) and B (role Y), mentioning role Y incorrectly wakes up bot A.

Fix: replace string matching with a set intersection against the bot's cached roles:

let is_role_mentioned = if !msg.mention_roles.is_empty() {
    let cached_roles = self.bot_role_ids.read().await;
    msg.mention_roles.iter().any(|r| cached_roles.contains(&r.get()))
} else {
    false
};

Technical notes:

1. Why guild_create instead of ready? ready.guilds contains UnavailableGuild stubs. Guild member data may not be available yet, particularly for larger guilds, causing .member() lookups to 404. guild_create fires per guild after ready with complete data.

2. Global HashSet vs per-guild HashMap: We use a flat HashSet<u64> for the cache. Discord Role IDs are globally unique and msg.mention_roles only contains roles valid for the current guild, so a flat cache is safe and keeps lookups simple.

3. Known limitation: If a role is added to the bot while it is running, it won't be recognized until restart. A guild_member_update handler could keep the cache hot if dynamic role assignment becomes a requested feature.

Files changed

File	Change
src/discord.rs	Add bot_role_ids: Arc<RwLock<HashSet<u64>>> to Handler; replace string-match with cache lookup; add guild_create handler to populate cache
src/main.rs	Initialize empty bot_role_ids cache

Test plan

• cargo check passes ✅ (verified locally)
• Single-bot guild: role mention still triggers the bot
• Multi-bot guild: role-mentioning bot A's role does NOT trigger bot B
• Bot has no guild roles: role mention does not trigger the bot
• Large guild: role cache populates correctly via guild_create
• No role mentions in message: early-exit, no cache read