feat(cli): active-context model, use/context, header & logo (spec 30 phase 1)

[gustavobertoi]

Summary

First implementation slice of the interactive-DX lane (spec 30 / D18). Introduces an active "current workspace + project" and the console/branding polish around it. No changes to the deterministic generate pipeline or lock-first concurrency.

What's in

• State — migration v4 adds active_context (one row per Docker context, FK/CASCADE like every ledger table) + CRUD SetActiveContext/ActiveContext/ClearActiveContext. Append-only/forward-only.
• Resolution — resolveActiveProject: --project → DEVSTACK_PROJECT → persisted active (only when its workspace_root matches the discovered workspace) → single/first project. Wired into defaultProject and the messaging default seam, so all db/s3/queue/topic/stream/resource commands inherit it. No behavior change when nothing is set.
• Commands — devstack context (+ --json) prints the resolved workspace/project/role/docker-context/backend/version; devstack use [name] persists the active project (or switches to a registered workspace) under the flock, and --print emits an eval-able cd+export DEVSTACK_* script for the shell wrapper (Phase 2).
• Console header — renderContextHeader atop status, gated off under --json/--quiet (the update-notice precedent).
• Branding — internal/branding embeds an ASCII logo, shown at the top of root help via Long (fang v2.0.1 exposes no logo option — verified; SetHelpFunc is overridden by fang, so Long is the injection point).

Tests / gates

• New: internal/state/active_test.go (CRUD + Docker-context isolation), internal/cli/context_test.go (registration + resolution precedence incl. stale-root and env cases).
• make ci (fmt-check + vet + build + test-race) and make determinism both green.

Scope / follow-ups (next phases of spec 30/31)

• Phase 2: shell-init eval hook (zsh/fish), completions install, prompt segment, use fuzzy-picker TUI.
• Phase 3+: project TUI, env/secrets TUI, JS/monorepo templates, devstack run task graph.

Open questions carried from the spec: [Q-CTX-SCOPE] (per-Docker-context vs global — implemented per-context).

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