feat(slack): UI + API to change the DM-default agent per workspace

[pavshulin]

Summary

When a Slack workspace has multiple agents bound to it, DMs to the bot are routed to a single "DM-default" agent. Today there is no way to choose or change which agent that is from the UI or any API — the flag is auto-set on the first agent ever connected to the workspace and is permanent unless an admin edits the SQLite database directly.

Current Behavior

The DM router (adapters/slack_adapter.py:201-240) uses this priority:

1. Channel binding (channel_id → agent) — works for non-DM channel messages
2. DM default agent (is_dm_default = 1) — used when message is a DM
3. Single connected agent fallback — works only if exactly one agent in workspace
4. Legacy slack_link_connections lookup
5. None

The is_dm_default flag on slack_channel_agents is set automatically in three places (all use is_first = len(agents_in_workspace) == 0):

Location	When
routers/slack.py:186	OAuth callback for first agent connected to a workspace
routers/slack.py:296	First agent bound via POST /api/agents/{name}/public-links/{link_id}/slack/connect
routers/slack.py:474	First agent bound via POST /api/agents/{name}/slack/channel

After that point, no setter exists. SlackChannelPanel.vue:33 only renders (DM default) as a label — no toggle, no dropdown.

Why This Matters

• Most common scenario — owner connects agent A first to test, later adds agent B/C/D. All workspace DMs continue going to agent A even when A isn't the right destination.
• Operational pain — only workaround is docker exec trinity-backend python3 -c "..." against /data/trinity.db, run by an operator with shell access. There's no audit trail and no operator-facing affordance.
• Unbinding leaves zero defaults — unbind_slack_agent deletes the row but doesn't promote anyone. Workspace ends up with no DM default; routing falls to step 3, which only works with exactly one remaining agent.

Proposed Scope

1. DB layer (db/slack_channels.py)

Add a single-transaction setter:

def set_slack_dm_default(self, team_id: str, agent_name: str) -> bool:
    """
    Make agent_name the DM-default for the workspace.
    Returns True if a row was updated, False if agent_name isn't bound there.
    """
    with get_db_connection() as conn:
        cur = conn.cursor()
        cur.execute("BEGIN")
        cur.execute(
            "UPDATE slack_channel_agents SET is_dm_default = 0 WHERE team_id = ?",
            (team_id,),
        )
        cur.execute(
            \"\"\"UPDATE slack_channel_agents SET is_dm_default = 1
                 WHERE team_id = ? AND agent_name = ?\"\"\",
            (team_id, agent_name),
        )
        changed = cur.rowcount > 0
        conn.commit()
    return changed

The clear-then-set inside one transaction avoids a transient "two defaults" state.

Also: when unbind_slack_agent removes the current default, promote the oldest-bound remaining agent (ORDER BY created_at ASC LIMIT 1) to keep the workspace addressable.

2. Backend API

New endpoint, owner/admin-only:

PUT /api/agents/{name}/slack/channel/dm-default

200 → {"workspace": "...", "team_id": "...", "previous": "agent-a", "new_default": "agent-b"}
404 → agent not bound in any workspace
403 → caller is not the owner / admin

Audit-logged via platform_audit_service.log with event_type=AGENT_LIFECYCLE, event_action="slack_dm_default_changed", target_id=agent_name, details={"team_id": ..., "previous": ..., "new_default": ...}.

3. MCP tool (optional)

Add set_slack_dm_default(agent_name) to src/mcp-server/src/tools/slack.ts (if a Slack tools module exists; otherwise skip — UI is the primary surface).

4. Frontend

src/frontend/src/components/SlackChannelPanel.vue:

• Replace the read-only (DM default) label with a dropdown listing all agents in the workspace.
• Selection PUTs the new default; success → toast, refresh.
• Disabled for non-owner / non-admin.

If only one agent is bound, the dropdown is suppressed (no choice to make).

Suggested Tier / Categorization

• Tier: P1b — owner-facing capability gap, not blocking but ergonomically painful
• Epic: fix(slack): channel user identity, observation mode, and proactive messaging #350 Slack Identity (this is exactly its scope — "who answers in Slack")
• Theme: Channels

Acceptance Criteria

• DB method set_slack_dm_default(team_id, agent_name) with single-tx clear-then-set
• unbind_slack_agent auto-promotes oldest-bound remaining agent if it removed the default
• PUT /api/agents/{name}/slack/channel/dm-default endpoint, owner/admin-gated, audit-logged
• SlackChannelPanel.vue shows a dropdown for the DM default; works end-to-end
• Unit test: set_slack_dm_default is idempotent and exclusive (no two-defaults race)
• Unit test: unbind_slack_agent promotes correctly when removing the default
• Manual smoke: connect 2 agents, flip default via UI, send DM, confirm routing changes

Out of Scope

• Per-channel routing changes (that already exists via channel bindings)
• A new "DM router" with rules — just a single default agent
• Migrating away from the is_dm_default boolean schema

References

• Routing logic: src/backend/adapters/slack_adapter.py:201
• DB layer: src/backend/db/slack_channels.py:138-198
• Frontend label: src/frontend/src/components/SlackChannelPanel.vue:33
• Auto-set sites: src/backend/routers/slack.py:186, 296, 474
• Local runbook for the DB-direct workaround: .local/notes/runbook-slack-dm-default-flip.md

[pavshulin]

/claim

[github-actions]

Claimed by @pavshulin. Label status-in-progress added.

Next steps:

1. Create branch: feature/584-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #584