feat(discord): register slash commands + interaction reply path

[tarrencev]

Summary

Discord bots now register seven built-in slash commands on startup via ApplicationCommandsBulkOverwrite:

• Meta — /ask <prompt> [private], /reset, /status, /stop, /help
• Agent-backed tool shortcuts — /recall <query> (memory search), /summarize [count]

Bulk-overwrite replaces the full application command list atomically, which means stale commands from any prior backend using the same Discord application are removed automatically on the first sidecar boot — no separate cleanup step required.

Reply path

/ask, /recall, /summarize defer their ACK and reply via the Discord interaction-token flow: the first chunk edits the deferred response (so the reply surfaces inline against the slash command in Discord's UI), subsequent chunks become interaction followups. Tokens expire 15 min after the invocation — if the agent run runs past that cap, the outbound dispatcher auto-falls back to a regular ChannelMessageSend with a warning log, so the user still gets the reply.

/reset, /status, /stop, /help are inline + ephemeral (visible only to the invoker). /reset and /stop are v1 stubs that acknowledge the request and point at what will land in a follow-up PR (a SessionClearer dependency + a scheduler-cancel hook on channels.Manager).

Design decisions baked in

• Global by default — commands register application-wide (~1h propagation). discord.test_guild_id escape hatch re-registers per-guild (instant) for dev iteration.
• /ask public by default, flips to ephemeral via its private:bool option. Other commands always ephemeral.
• No /thread yet — that command depends on the create_discord_thread tool (upstream PR feat(tools): add create_discord_thread tool #976). This PR stays self-contained against upstream/dev; /thread lands as a small follow-up once feat(tools): add create_discord_thread tool #976 merges.

Mechanism

Mirrors the existing Telegram SyncMenuCommands retry pattern (internal/channels/telegram/channel.go:223-248): 3 attempts, linear backoff, runs in a goroutine so a transient Discord 5xx doesn't break channel startup. The interaction token plumbing uses routingMetaKeys so the token survives the inbound→agent→outbound hop; Discord Send() branches on msg.Metadata["discord_interaction_token"] to choose between the interaction path and the regular channel post.

Config

discord: {
  // default true — opt out:
  slash_commands: false,

  // optional — dev escape hatch:
  test_guild_id: "1234567890",
}

Requires the applications.commands OAuth scope on the bot's installation. A missing scope shows up at startup as discord: failed to fetch application ID (slash commands disabled) — the channel continues to run, just without slash commands.

Test plan

• go build ./... + go build -tags sqliteonly ./... + go vet ./... clean
• go test ./internal/channels/discord/... — new tests pass
  • Command schema characterization (pins command set + required-option shape)
  • SyncSlashCommands bulk-overwrite dispatch (global + guild-scoped + missing-app-id + API-error + stale-clearance)
  • chunkForDiscord chunking rules (empty, fits, split-at-newline-in-second-half, hard-split fallback)
  • interactionEcho.expired 15-min lifetime
  • optionString / optionBool / optionInt helpers
  • buildAgentPrompt for /ask / /recall / /summarize incl. private flag + count clamp
  • routingMetaKeys includes discord_interaction_* keys
• Manual: invite bot to test guild with test_guild_id set, confirm /ask, /recall, /summarize, /help each work end-to-end and that stale commands from the prior backend vanish on first boot.

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