fix: headless init does not stamp project time_initialized so memory stays inert

[LeXwDeX]

Closes#404

Root cause (runtime-proven)

Headless /init (opencode run, serve POST /session/:id/message text part) delivered "/init" as plain text; slash-command routing existed only in the TUI client (client-side parse → POST /session/:id/command). SessionPrompt.command never ran → Command.Event.Executed never fired (sole publish site: prompt.ts:2144) → project.time_initialized stayed NULL → MEMORY fail-closed inert.

Fix

Server-side routing in handlers/session.ts: parseCommandPayload + promptOrCommand route single-text-part prompts that name a registered command through promptSvc.command; registry miss falls through to the plain prompt (existing semantics preserved). Both prompt and promptAsync endpoints covered. TUI /command handler byte-identical (additive change only).

Verification (verify node, independently reproduced)

• RED proof at parent commit: repro arms 1 (run "/init") + 4 (serve message text) fail; controls (run --command init, serve /command) pass
• GREEN on fix commit: 4/4 arms pass; E2E from-source run stamps non-NULL
• Gates: typecheck 0 · lint 4838/4850 (zero new) · test:dag-core all floors · command/session suites 25/25