RFC: Add capacity-block-and-training-plan-finder skill

## Summary

Add a skill that finds reservable AWS accelerator capacity by scanning **EC2 Capacity Block** offerings (`ec2:DescribeCapacityBlockOfferings`) and **SageMaker Training Plan** offerings (`sagemaker:SearchTrainingPlanOfferings`) across regions and instance types for a given count, duration, and start date. It fans out region×instance-type lookups in parallel, isolates per-region errors, falls back to halved count/duration when nothing is found, and returns available offerings (region, AZ, start/end dates, upfront fee, number of parts) as a ranked table.

It is derived from an existing working AWS sample — a single-file Streamlit app (`sample-capacity-finder-for-ec2-capacity-block-and-sagemaker-training-plan`) — repackaged as an agent-invokable skill.

## Motivation

Startups training and fine-tuning models depend on scarce GPU/Trainium capacity (P4/P5/P6, Trn1/Trn2). Finding it today means clicking through the console region-by-region across two "find 8× p5.48xlarge for 14 days starting next Monday" in one natural-language step — read-only, across all supported regions at once — which fits the startups audience racing to secure training capacity.

## Proposed Solution

- A `SKILL.md` plus a small Streamlit-free Python helper that reuses the two boto3 calls.
- Inputs: instance types, instance count, regions (or "all"), duration in days (mapped to hours), start date, optional end date.
- Parallel scan via a bounded thread pool, per-region error isolation, and an automatic halved-parameter fallback when the initial search returns nothing.
- Strictly read-only IAM: `ec2:DescribeCapacityBlockOfferings`, `sagemaker:SearchTrainingPlanOfferings`.
- Output as a table, with optional structured JSON for chaining (see Open Questions).

## Alternatives Considered

- **Keep it only as the standalone Streamlit app** — great for humans, but not invokable as an agent skill and not distributable through this marketplace.
- **New dedicated plugin vs. folding into an existing plugin (e.g. `aws-dev-toolkit`)** — either works; deferring the routing decision to maintainers (see Open Questions).

## Open Questions

- **Which plugin should host this skill** — an existing one such as `aws-dev-toolkit` (`solution-architecture/`), or a new standalone plugin with its own `marketplace.json`entry? Which team folder and CODEOWNERS should own it?
- Should output be a table only, or also emit structured JSON so results can be piped into downstream automation (e.g. a follow-on reservation step)?
- How should the supported instance-type and region lists be maintained over time?

## Drawbacks

- The offerings APIs evolve and their results vary by account entitlement, so responses can be empty or misleading without context — the skill must explain this.
- The instance-type and region lists need occasional maintenance as new hardware launches.