Improve Discord exec approval summaries

[ryouchida715]

Summary

• Add a short human-facing Japanese summary at the top of Discord exec approval prompts.
• Explain what the command is about, why approval is needed, and how the user should decide.
• Localize Discord exec approval button labels to Japanese: 許可 / セッション許可 / 常に許可 / 拒否.

Why

In Discord, dangerous command approval prompts currently show the raw command and "Reason" first. For non-engineer users this makes it unclear what action is being requested. This change keeps the command visible, but puts a short actionable explanation first.

Test Plan

• python -m pytest tests/gateway/test_discord_exec_approval_summary.py -q -o 'addopts=' → 3 passed
• python -m py_compile plugins/platforms/discord/adapter.py tests/gateway/test_discord_exec_approval_summary.py → exit 0
• git diff HEAD --check → exit 0

Notes

• No gateway restart or deployment has been performed by this PR.
• No secrets or credentials are included in the diff.

[teknium1]

Closing as superseded: #60245 restructured send_exec_approval so the full command (up to ~1900 chars) is now visible in plain message content next to the buttons, which addresses the readability problem this PR targeted. The head/tail elision idea was a nice touch but conflicts with the new layout. Thanks for the contribution!