fix(discord): notify owners on blocking prompts

[doryani-ai]

Summary

Discord blocking prompts can now opt in to mentioning numeric allowlisted owners, so an agent waiting in a quiet channel or thread produces a native notification instead of silently stalling.

This is a current-main salvage of #47342 by @nanobro. It preserves that PR's four-prompt scope while incorporating the July review feedback: no Discord-only kwargs cross the generic gateway contract, mentions live in the plain content mirror, and delivery uses the existing opt-in discord.approval_mentions mechanism with scoped AllowedMentions.

Changes

• extend discord.approval_mentions from exec approvals to:
  • slash-command confirmations
  • clarify questions (button and open-ended paths)
  • update prompts
• keep the setting default-off and mention only numeric entries from discord.allow_from
• keep role, @everyone, replied-user, and non-owner user pings disabled by
  passing the exact numeric owner objects to Discord's AllowedMentions
• keep each adapter's opt-in profile-scoped under multiplexing while preserving
  the documented env-over-config precedence in single-profile mode
• bound large owner mention lists and account for mentions inside Discord's
  2,000-character content budget
• document the complete setting behavior in the Discord user guide

Validation

• regression tests were written first and failed on all three missing sibling paths
• canonical targeted runner: 29 passed across mention, profile-isolation,
  content-budget, content-mirror, clarify, allowed-mentions, approval-content,
  and timeout coverage
• ruff check passed for the changed Python files
• static added-line scan found no credential, injection, eval/exec, unsafe deserialization, or SQL-construction findings

Prior art

• fix(discord): mention user in approval/clarify prompts for notification ping #47342 identified the exact user-visible problem and four blocking prompt paths; this update credits and salvages that work rather than treating it as novel.
• feat(discord): opt-in owner mentions on exec-approval prompts #60493 established the accepted opt-in content-mirror and scoped-mention mechanism for exec approvals; this change extends that mechanism to the three known siblings.

No issue is closed: the exact existing contribution is #47342.