feat(skill): cost-aware batch workflow + agentkey_account in tools table

[f0rmatting]

Summary

Teaches the LLM the batch-confirmation workflow that PR chainbase-labs/AgentKey-Server#70 (`describe_tool` cost field) makes possible.

Trigger: `≥3 calls` OR estimated cost `≥10 credits`.

Workflow: `describe_tool` → read `cost.credits_per_call` → multiply by N → `agentkey_account` for balance → present plan + estimate + balance to user → wait for confirmation → execute.

Layer separation

• SKILL.md (Layer 1): gained 5 lines total — one row in the MCP Tools table for `agentkey_account` (marked "Free" + graceful fallback note), one rule under Rules pointing to the reference, one clause in step 0.C clarifying the 4 core tools are required and `account` is optional. No long-form prose.
• `references/cost-aware.md` (Layer 2, new, ~870 tokens): full workflow — pre-batch checklist, how to read `describe_tool`'s cost field (three shapes: number+map, billing_note only, free), user-confirmation script, cost-saving moves (provider switch, probe-first, dedup, trim N), balance-check failure recovery.

Lazy-loaded — the reference only enters the model's context when the trigger fires.

Backwards compat

• Old servers without `agentkey_account` exposed via MCP: 0.C does not gate Setup on its presence; the reference doc's "balance check failed" branch covers the case.
• Existing 4-tool clients keep working unchanged.

Test plan

• Install the local branch (`npx skills add .`) and ask Claude something obviously bulk: "find 5 xiaohongshu creators in skincare and get each one's profile" — expect the agent to call `describe_tool`, then `agentkey_account`, then present the plan before executing.
• Ask a one-shot lookup ("search for AI news"): expect the agent to skip the workflow per the "single call + ≤1 credit + no batch intent" carve-out in the reference.
• Ask against a server without `agentkey_account`: expect the agent to proceed without setup-loop failures.

Release

No `version.txt` bump — release-please will pick up the `feat:` prefix and open the bump PR. Per repo CLAUDE.md, plugin updates trigger on GitHub Release publication, not on commits, so this lands quietly until the next cut.