Harness 3.1 Errors users can act on [backend + inspector, 2 PRs] #6543

Part of #6535.

• Rewrite every user-visible harness error with the ui-copy skill:
  • Say "client", not "host".
  • Don't name env vars or config fields.
  • Use display names, not raw harness ids.
  • Say what to do next.
• Strings:
  • server/utils/harness/harness-availability.ts: ~304, ~395, ~441-460, ~643, ~656, ~673
  • harness-box.ts ~94
  • local/request-target.ts ~88
  • local/availability.ts ~192
  • server/routes/mcp/chat-v2.ts: ~1485, ~1505, ~1511, ~1612
  • client/src/lib/chat-send-request.ts ~98
  • backend featureGates.ts ~425, ~702-714
  • backend harnessLeaseStartGates.ts ~71, ~81
• Free tier: the refusal (harnessLeaseStartGates.ts ~66-75) and the CTA in client/src/components/chat-v2/error.tsx (~129) offer personal BYOK. Harness clients accept only MCPJam credits or an org key, so offer upgrade or an org key instead. Add a free_tier_model_restricted branch.

Delivery rules

• Fresh worktree and branch; each PR at most 400 handwritten changed lines; stack when necessary.
• Verify the actual failing boundary and run repository checks.
• Preserve broker-only model auth, no venue/engine fallback, and separate infrastructure failures.
• Human-only: production deployment, flags/env changes, rebuilds/backfills, signed/published packs and large paid runs.

Status: planned; completion requires the acceptance evidence above.

Pull request stack

• MCPJam/mcpjam-backend#2204: fix(harness): make access and funding errors actionable
• MCPJam/inspector#6590: fix(harness): make setup and billing refusals actionable
• MCPJam/inspector#6606: fix(harness): preserve actionable API and assistant errors

The implementation hub records merge order, current verification and remaining release gates. PR publication does not mark the deployed or packaged acceptance matrix complete.