Migrate license from Apache-2.0 to MIT

[pentaxis93]

Context

Groundwork is Apache-2.0 but composes with MIT-licensed upstream skills (Superpowers). The Apache-2.0 machinery — patent grants, NOTICE propagation, explicit contribution terms — serves multi-contributor corporate projects with patentable technology. Groundwork is a single-contributor methodology project where none of that applies.

The license choice also drives the ATTRIBUTION.md overhead and "Groundwork Originals" branding identified in #5.

What Apache-2.0 provides over MIT

1. Explicit patent grant — self-to-self with one contributor
2. Patent retaliation clause — no patentable surface in methodology skills
3. NOTICE file propagation — the attribution machinery we're trying to shed
4. Explicit contribution terms — implicit under MIT via standard PR workflow

Change

• Replace LICENSE contents with MIT license text
• Update Cargo.toml license field(s) from Apache-2.0 to MIT
• Replace ATTRIBUTION.md with minimal upstream acknowledgment (Superpowers source, author, license, pinned revision) — no branding, no performative framing
• Update any SPDX-License-Identifier headers if present

Acceptance criteria

• LICENSE contains MIT text with correct copyright
• All Cargo.toml files specify license = "MIT"
• No "Apache" references remain in project metadata
• Upstream Superpowers attribution preserved (source, author, license, revision)
• cargo test -p groundwork-cli passes

Subsumes the attribution/branding scope of #5.