feat(server): Slack mentor entry points (App Home, slash command, CTA)

[FelixTJDietrich]

Part of #1204.

What ships

Three Slack-side entry points into a mentor DM thread, all routing through the same SlackChannelAdapter from #1256 and the same orchestrator the web mentor uses:

• App Home "Open mentor" CTA. The App Home tab from feat(server): Slack App Home tab as primary onboarding surface #1248 grows a primary action that opens a DM thread to the bot for the viewing user. The CTA resolves the user's most recent mentor_session of surface SLACK_DM (via feat(server): mentor_session surface discriminant + shared user memory #1242) and either resumes or starts fresh.
• Slash command /hephaestus mentor. Registered on the per-workspace Slack app from refactor(server): per-workspace Bolt App factory replacing the singleton #1244. Invocation in any channel responds ephemerally with a "opening a DM" acknowledgement and triggers the same mentor session as the App Home CTA.
• App Home resume state. When the user opens App Home and an in-flight SLACK_DM mentor_session exists for their (user_id, workspace_id), the tab renders a resume card with the session's last summary; clicking it deep-links the DM thread.

All three entry points require the user to have a linked Slack identity (identity_link row with provider=SLACK, team_id=<team>, external_id=<slack_user_id> from #1238). Unlinked users see the nag flow from #1263 instead.

Why

A single, predictable entry surface is what makes the Slack mentor land. Three entries cover the three muscle memories: power users hit a slash command, light users open App Home, and the resume state catches anyone who lost their DM thread.

Acceptance criteria

• App Home "Open mentor" CTA renders for linked users; clicking it opens a DM via conversations.open and triggers the orchestrator with surface=SLACK_DM
• /hephaestus mentor is registered on the per-workspace Slack app; invocation in any channel returns an ephemeral 200 within 3 s and the DM thread arrives within 5 s
• An in-flight SLACK_DM mentor_session for the viewing user renders a resume card on App Home with the session's last summary; clicking deep-links the DM thread
• Unlinked users invoking any of the three entries trigger the nag flow from feat(server): identity-link nag flow with cap and stop button #1263 (a one-time linkage DM + the workspace daily-cap-counted reminder), never a mentor turn
• An integration test installs the Slack app for two workspaces, opens App Home as a linked user in each, and asserts the DMs route to independent Bolt App instances (no cross-workspace leakage)
• All three entry paths honor the kill switch from feat(server): DM safety rails for proactive mentor sends #1259; with the switch off, the slash command returns "Mentor temporarily unavailable" and the App Home CTA is hidden

Tests to write

• SlackMentorEntryPointsIT — three test scenarios, one per entry, asserting the DM is opened and the orchestrator is triggered with the right surface.
• A test for the unlinked-user → nag-flow path (composes with feat(server): identity-link nag flow with cap and stop button #1263).
• A kill-switch-off test for the slash command response surface.

Implementation notes

• The slash command's 3-second response budget is non-negotiable; the handler returns the ephemeral 200 immediately and the DM open is enqueued. The same pattern Slack OAuth install uses in feat(server): Slack OAuth install flow with Bolt InstallationService #1243.
• App Home rendering is per-view; the App Home tab from feat(server): Slack App Home tab as primary onboarding surface #1248 is the host, and this sub-issue adds the mentor card to it. The card's data fetch reads mentor_session directly via the SPI, not via a webapp endpoint.
• The resume card surfaces the session's last summary field (already populated by the orchestrator on each turn boundary). No new schema.
• Slash command argument parsing is out of scope; /hephaestus mentor is the only sub-command this epic ships. Future sub-commands extend the same router.

Dependencies

Depends on #1256. Depends on #1242. Depends on #1244. Depends on #1248. Depends on #1238. Blocks #1258.