Port mguttmann workflow engine stack from anomalyco/opencode

[dzianisv]

Port upstream workflow engine pieces needed for JS workflow scripts and /workflow command into fork branch stabilize-smoke-gate.

Scope:

• PR feat(workflow): engine-core (1/6) — modularized workflow engine, split from #29789 anomalyco/opencode#32390 engine core
• PR feat(workflow): server routes + SDK (2/6) — split from #29789 anomalyco/opencode#32392 server routes + SDK
• PR feat(workflow): ctx.tool + plugin registration (5/6) — split from #29789 anomalyco/opencode#32395 tool + plugin registration
• skip feat(workflow): TUI workflow dialogs (3/6) — split from #29789 anomalyco/opencode#32393 and feat(workflow): web app + desktop (4/6) — split from #29789 anomalyco/opencode#32394 for initial port

Goal: users can author JS workflow scripts via plugin workflow() helper and run them through /workflow.

[dzianisv]

Plan: manual-port workflow stack from upstream PRs anomalyco#32390, anomalyco#32392, anomalyco#32395. Cherry-pick anomalyco#32390 hit unrelated core conflicts, so taking PR32395 as source of truth, copying new workflow/server/tool/plugin files, merging minimal hooks into local routing/registry/CLI, then running targeted typecheck and workflow/server tests.

[dzianisv]

Investigated. The referenced PRs (anomalyco#32390, anomalyco#32392, anomalyco#32395) don't exist in anomalyco/opencode's public dev branch. No workflow-related commits found in upstream/dev. Either these PRs haven't been merged upstream yet or they're in a private branch.

Cannot port what doesn't exist yet. Leaving open — will revisit when upstream ships the workflow engine to dev.