RFC: Add cost-assessment skill (formal WA Cost Pillar assessment)

[gcanales75]

Summary

A new cost-assessment skill that runs a formal, structured Well-Architected
Cost Optimization Pillar assessment (COST 1–11) and produces a scored report —
the deeper counterpart to the tactical cost-check scan.

Motivation

Startups repeatedly need a documented, defensible cost baseline before
committing to Savings Plans / RIs or transitioning off Activate credits, which
cost-check (quick scan) and well-architected (all six pillars) don't cover.

Proposed Solution

A single SKILL.md under skills/cost-assessment/ using a read-only four-block
methodology (Discovery → Pricing & Savings → Demand & Supply → scored
Assessment), referencing the plugin's awspricing/awsknowledge tools, with a
"When to use this vs cost-check / well-architected" table to keep the boundary
explicit.

Alternatives Considered

Extending cost-check (rejected — would make the quick-scan skill heavier) or
folding into well-architected (rejected — that skill spans all six pillars).

Open Questions

Preferred name (cost-assessment vs wa-cost-review)? Reference aws-cost MCP
"when available" or standardize on shipped tools?

Drawbacks

Some adjacency to cost-check; mitigated by the differentiation table.