Rename project in README.md

[Basheirkh]

Updated project name from 'nilscript' to 'NIL Script' in README.

What & why

Type

• Editorial (docs/typo) — label editorial, fast-track
• Tooling / SDK / CLI (non-normative)
• Normative spec change — requires the process below

For normative changes (GOVERNANCE.md)

• An issue was opened first using the proposal flow, discussed ≥ 14 days
• Includes a §15 Security considerations analysis ("it's convenient" is not one)
• Implementation experience exists in the reference implementation
• No invariant weakened (closed performative set, Six Guarantees, floors, tenant isolation, preview completeness)

Checks

• pytest green locally
• Schemas/examples still validate (they run in CI)
• Docs/cross-links updated if behavior or commands changed