feat(discord): add exec approval forwarding to DMs (#1621)

* feat(discord): add exec approval forwarding to DMs

Add support for forwarding exec approval requests to Discord DMs,
allowing users to approve/deny command execution via interactive buttons.

Features:
- New DiscordExecApprovalHandler that connects to gateway and listens
  for exec.approval.requested/resolved events
- Sends DMs with embeds showing command details and 3 buttons:
  Allow once, Always allow, Deny
- Configurable via channels.discord.execApprovals with:
  - enabled: boolean
  - approvers: Discord user IDs to notify
  - agentFilter: only forward for specific agents
  - sessionFilter: only forward for matching session patterns
- Updates message embed when approval is resolved or expires

Also fixes exec completion routing: when async exec completes after
approval, the heartbeat now uses a specialized prompt to ensure the
model relays the result to the user instead of responding HEARTBEAT_OK.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat: generic exec approvals forwarding (#1621) (thanks @czekaj)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

docs/tools/slash-commands.md

@@ -61,6 +61,7 @@ Text + native (when enabled):
 - `/skill <name> [input]` (run a skill by name)
 - `/status` (show current status; includes provider usage/quota for the current model provider when available)
 - `/allowlist` (list/add/remove allowlist entries)
+- `/approve <id> allow-once|allow-always|deny` (resolve exec approval prompts)
 - `/context [list|detail|json]` (explain “context”; `detail` shows per-file + per-tool + per-skill + system prompt size)
 - `/whoami` (show your sender id; alias: `/id`)
 - `/subagents list|stop|log|info|send` (inspect, stop, log, or message sub-agent runs for the current session)