Support dynamic workflows (/workflows view) in BYOK / air-gapped sessions #5055

In BYOK/air-gapped sessions /workflows returns "Unknown command" (UI gated on GitHub user/quota data), and with COPILOT_OFFLINE=true the workflow tools are removed entirely — even though dynamic workflows otherwise run fine against a BYOK model.

Proposed solution

Make dynamic workflows fully usable when a BYOK provider is configured, without requiring a GitHub user/quota response:

1. Show /workflows (command, completion, and Workflows view) for BYOK sessions.
   Today visibility appears to be gated on the Copilot user response (EXTENSIONS && sessionQuotaProjection(sessionId).dynamicWorkflowsUiVisible), cached in ~/.cache/copilot/copilot-user-cache.json. With no cache (fresh/air-gapped machine) or a stale one (~25 min in my testing), the command disappears, while the underlying tools (run_dynamic_workflow, dynamic_workflows_manage) keep working. When a BYOK provider is active, the UI should follow the same availability as the tools.

2. Support dynamic workflows with COPILOT_OFFLINE=true.
   In offline mode the workflow and extension tools are not offered to the model at all (17 vs 25 tools in my test), so workflows are unusable. Authoring/running workflows needs no network when the model is local, so offline mode should keep them.

3. If gating is intentional, explain it. Replace Unknown command: /workflows with a message saying why it is unavailable and how to enable it.

Benefit: on-prem, regulated, and air-gapped users running local models via BYOK (documented as "GitHub authentication is not required") get the same workflow experience — run monitoring, pause/resume, progress — instead of a hidden feature.

Example prompts or workflows

Environment for all examples: air-gapped Linux host, local vLLM model via BYOK:

COPILOT_PROVIDER_BASE_URL=http://<local-host>:<port>/v1 \
COPILOT_MODEL=<local-model> \
copilot --experimental

1. /workflows → open the Workflows view and watch phases/agents of a running review workflow (today: ✗ Unknown command: /workflows).
2. "Create a workflow that reviews every changed file in parallel with a subagent per file, then summarizes findings" → author + run, then monitor it in /workflows.
3. "Resume workflow run with a higher maxTotalSubagents" after a limit was hit, from the Workflows view.
4. COPILOT_OFFLINE=true copilot --experimental → "What dynamic workflows are available?" (today: the workflow tools are not available in offline mode).

Additional context

• Copilot CLI 1.0.91, Linux x86_64, bash, VS Code integrated terminal.
• Reproduction (no real air gap needed): fresh COPILOT_HOME, HTTPS_PROXY=http://127.0.0.1:9 NO_PROXY=127.0.0.1,localhost, a local OpenAI-compatible endpoint, then type /workflows.

• Related BYOK subagent issues that also limit workflows: Allow subagents.maxConcurrency for BYOK / non usage-based accounts (currently hard-capped at 2) #4940 (concurrency hard-capped at 2), 🐛 Repo .claude/agents/*/AGENT.md model: field overrides session model for Copilot custom agents, breaking subagent spawns on BYOK providers #4437 (agent model: overrides break BYOK subagent spawns).