docs: use em dash in license line

[staging-devin-ai-integration]

Summary

Trivial typographic fix: replaces the ASCII hyphen-minus (-) with an em dash (—) in the License section of README.md.

This PR was created to verify the declarative environment setup (clone → install → lint → test → commit → push → PR).

Environment verification results:

• just lint-ui ✔ (prettier, eslint, tsc)
• just lint-skit ✔ (cargo fmt, clippy, cargo-deny, reuse)
• UI tests: 863 passed (vitest)
• Rust tests: 498 passed, 21 GPU-only failures (expected — no GPU adapter in headless VM)

Review & Testing Checklist for Human

• Confirm the em dash renders correctly in the GitHub README preview

Notes

Green-risk change — single character substitution in a markdown file.

Link to Devin session: https://staging.itsdev.in/sessions/ee13b35fdfcf4a47a1de55704c727527
Requested by: @streamer45

[staging-devin-ai-integration]

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