feat(slack): UI + API to change DM-default agent per workspace (#584)

[pavshulin]

Summary

Closes #584. Until now the Slack is_dm_default flag was auto-set on
the first agent connected to a workspace and unchangeable through
any UI or API — the only workaround was direct SQLite edit. Multi-agent
workspaces were stuck routing every DM to the first agent forever.

This PR adds the missing setter and tightens unbind semantics so the
workspace can never end up with no DM default.

Changes

Backend

• db/slack_channels.py — new set_dm_default(team_id, agent_name) uses single-tx clear-then-set inside one BEGIN/COMMIT to avoid any window with two defaults (schema has no exclusivity constraint).
• routers/slack.py — PUT /api/agents/{name}/slack/channel/dm-default: owner-gated via can_user_share_agent, audit-logged via AGENT_LIFECYCLE/slack_dm_default_changed (details: team_id, workspace_name, previous, new_default). Idempotent — returns status=unchanged if caller is already default.
• routers/slack.py — DELETE /slack/channel now returns 409 if the target is the workspace's DM default and other agents are bound. Promoting a sibling first is the only path past the guard. When the target is the only agent, unbind is allowed (clean cascade).
• routers/slack.py — GET response now includes workspace_agent_count so the UI can decide whether to disable Unbind.

Frontend

• SlackChannelPanel.vue — replaces the read-only (DM default) label with a "Make default" button (when not default) or a "✓ DM default" badge (when default). Both carry a hover tooltip explaining DM-default semantics. Unbind button is disabled with an explanatory tooltip when this agent is the DM default and siblings remain.

Tests — 10 unit tests, all pass:

• set_dm_default: returns False when agent unbound, sets exclusively, idempotent, isolated per workspace, doesn't touch siblings (5 tests)
• unbind_agent: pure delete (does NOT auto-promote — router enforces guard), works on non-default + last-agent paths, unknown agent returns False (4 tests)
• Plus migration/setup smoke (1 test)

Docs

• feature-flows/slack-channel-routing.md — endpoint table, UI state, DM-default semantics
• feature-flows.md index — Recent Updates row added
• tests/registry.json — entry for the new test file

Live verification on local dev

• ✅ DM routing flips immediately after PUT /dm-default (no restart, no cache — get_slack_dm_default_agent() queries DB on every call)
• ✅ Channel @mention routing unaffected (channel binding wins over DM default)
• ✅ 409 surfaces with the explanatory message when blocked
• ✅ audit_log row written with correct actor_type=user, event_action=slack_dm_default_changed, full details
• ✅ Verified end-to-end via direct call against SlackAdapter.get_agent_name() — flipping is_dm_default via API changes which agent is returned for an is_dm=True NormalizedMessage

Test plan

• pytest tests/unit/test_slack_dm_default.py -v — 10/10 pass
• Regression: pytest tests/unit/test_agent_shared_files_migration.py tests/unit/test_file_sharing_mixin.py tests/unit/test_file_download_no_session_gate.py -q — 39/39 pass
• Manual: flip via API, verify routing changes, verify 409 guard, verify audit row written
• Reviewer: /validate-pr 585 before merge
• Reviewer: confirm UI behavior on the Sharing tab (button visible, tooltip readable, disabled state correct when this agent is DM default with siblings)

Out of scope (deferred)

• MCP tool wrapping the same operation (issue marked it optional)
• Per-channel routing customization beyond the DM default (existing channel bindings already cover that)
• Migration away from the is_dm_default boolean schema

Related

• Issue: feat(slack): UI + API to change the DM-default agent per workspace #584
• Epic: fix(slack): channel user identity, observation mode, and proactive messaging #350 Slack Identity (would be the first ship in this epic — currently 0/9)
• Theme: Channels
• Tier: P1b

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