#15663: fix(discord): normalize DM session keys for multi-agent setups 🤖

## Summary Fixes #15656 — Discord DM messages to non-default agents are silently dropped in multi-agent setups after v2026.2.12. > **🤖 AI-Assisted PR** — Code written with Claude (AI), reviewed and tested by human on a production multi-agent deployment. Degree of testing: **fully tested in production** with 3 agents (main, work, fina) on separate Discord bot accounts. ## Problem When using `per-channel-peer` dmScope with multiple agents bound to separate Discord bot accounts, DMs to non-default agents (e.g., `fina`, `work`) create phantom session entries with the wrong key format: - **Expected**: `agent:fina:discord:direct:USERID` - **Actual**: `agent:fina:discord:channel:USERID` (no model, no session file → silently dropped) ### Root Cause `parseDiscordTarget(target, { defaultKind: "channel" })` in `resolveDiscordSession()` treats bare numeric user IDs as channel IDs. When outbound session routing processes a DM reply, it generates `discord:channel:USERID` keys. These phantom session entries get persisted via `ensureOutboundSessionEntry()` with no model and no session file, and subsequent messages get routed to them instead of the correct `discord:direct:USERID` sessions. Additionally, the session key format changed from `discord:dm:` to `discord:direct:` between versions with no migration path for existing sessions. ## Fix In `resolveSessionKey()` (`src/config/sessions/session-key.ts`), when `chatType` is `"direct"`, normalize misrouted Discord session keys: 1. **Legacy migration**: `discord:dm:USERID` → `discord:direct:USERID` 2. **Phantom fix**: `discord:channel:USERID` → `discord:direct:USERID` (only when sender ID matches the ID in the key, to avoid accidentally rewriting real channel keys) This is a targeted, safe fix at the session key resolution layer. The sender ID guard ensures real `discord:channel:` keys for guild channels are never rewritten. ## Testing - [x] Unit tests added (`session-key.test.ts`) covering all normalization cases and edge cases - [x] Tested on production instance with 3 agents, 3 Discord bot accounts, `per-channel-peer` dmScope - [x] Verified: main agent DMs still work, fina/work agent DMs now work, guild channel messages unaffected ## Notes A more comprehensive upstream fix could also make `parseDiscordTarget` context-aware about DM vs channel targets, but that would be a larger change with wider blast radius. This PR takes the minimal safe approach.