fix(dashboard): trust sessions endpoint for status

[wtchronos]

Summary

• Fetch dashboard Sessions Intelligence data directly from /api/sessions instead of trusting the broader gateway-status capability probe.
• Preserve the real unavailable state when the sessions route returns source: 'unavailable' or code: 'capability_unavailable'.
• Update capability-unavailable copy to point at feature-specific local Workspace probes.
• Add focused tests for sessions response normalization and feature-gate probe copy.

Test plan

• pnpm vitest run src/lib/feature-gates.test.ts src/screens/dashboard/lib/sessions-query.test.ts
• pnpm build
• git diff --check

Notes

• Full pnpm test currently has unrelated baseline failures in MCP hub search, gateway capabilities env handling, context usage exports, i18n labels, chat context controls/message-list, Swarm2 copy contract, and HermesWorld localStorage tests. The focused tests added here pass.

[outsourc-e]

Closing as superseded by #432. The validated fix was folded into the consolidated batch branch fix/issue-sweep-batch-20260514 for one review/merge path.