bug(discord): streaming edit lost in 0.8.0 — DiscordAdapter missing edit_message and use_streaming overrides

[Agent-Mouses]

Description

After upgrading from 0.7.5 to 0.8.0, all Discord bots stopped streaming replies. Messages now appear as a single send-once block instead of live-editing as tokens arrive.

Root cause: impl ChatAdapter for DiscordAdapter in src/discord.rs no longer overrides edit_message() or use_streaming(). The trait defaults in src/adapter.rs return false / Err("not supported"), so Discord falls back to send-once mode.

In 0.7.7, edit_message() was present at line 57 of src/discord.rs. In 0.8.0, the entire impl block ends at line 109 without either method.

Likely regression from the Slack streaming refactor in #444.

Steps to Reproduce

1. Upgrade any Discord bot from 0.7.x to 0.8.0
2. Send a message that triggers a long reply
3. Observe: reply appears all at once instead of streaming (edit-in-place)

Confirmable via source diff:

# 0.7.7 — has edit_message
curl -s https://raw.githubusercontent.com/openabdev/openab/openab-0.7.7/src/discord.rs | grep -n edit_message
# 57:    async fn edit_message(...)
# 61:            .edit_message(...)

# 0.8.0 — missing
curl -s https://raw.githubusercontent.com/openabdev/openab/openab-0.8.0/src/discord.rs | grep -n edit_message
# (no output)

Expected Behavior

Discord replies should stream via edit-in-place (same as 0.7.x behavior). The DiscordAdapter should override:

• use_streaming() -> true
• edit_message() using serenity http.edit_message()

Environment

• OpenAB: 0.8.0 (ghcr.io/openabdev/openab:0.8.0)
• Platform: Discord (4 bots, all affected)
• Agent: kiro-cli 2.0.0 via ACP
• Host: macOS (Docker Desktop)
• Previous working version: 0.7.5 / 0.7.7

Screenshots / Logs

No errors in logs — the adapter silently falls back to send-once because use_streaming() returns false by default. The edit_message default returns Err but is never called since use_streaming() gates it.

[github-actions]

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[Agent-Mouses]

Updated the issue body to match the bug template fields (Description, Steps to Reproduce, Expected Behavior, Environment, Screenshots / Logs).

Could a maintainer please add the bug label? I don't have label permissions on this repo. Thanks!