Remove ambiguous 'environment' metadata attribute

[Unisay]

Summary

Removes the environment field from submission metadata as it has proven to be ambiguous and not useful to authors.

Problem

The environment metadata attribute suffered from several issues:

• Low adoption: Only 28.6% of submissions used it (6 of 21)
• Inconsistent semantics: Different compilers interpreted it differently:
  • Plinth: Documented compiler toolchain dependencies (plutus-tx, plutus-core, plutus-ledger-api)
  • OpShin: Documented runtime dependency (uplc library)
  • Aiken: Present but empty ("dependencies": {})
  • Plutarch/Scalus: Field entirely omitted
• Not used by tooling: No Haskell code or bash scripts read or process this field - validation was purely schema-based
• Ambiguous purpose: Unclear whether it should track compiler dependencies, runtime dependencies, or package lockfile versions

Changes

Schema Changes (2 files)

• submissions/TEMPLATE/metadata.schema.json - Removed environment property definition
• submissions/TEMPLATE/schemas-README.md - Removed environment documentation

Submission Data (9 files)

Removed environment object from:

• 4 Plinth submissions (fibonacci_naive_recursion, factorial_naive_recursion, ecd, two_party_escrow)
• 2 OpShin submissions (factorial_naive_recursion, fibonacci)
• 3 Aiken submissions (fibonacci_naive_recursion, fibonacci tailrec/prepacked)

Documentation (3 files)

• README.md - Removed environment from metadata example and reproducibility notes
• doc/submission-guide.md - Removed environment from metadata example
• doc/domain-model.md - Removed environment references from domain model

Impact

• Zero breaking changes: Field was optional and not processed by any code
• Simpler schema: Reduces confusion for submission authors
• Can be re-introduced later: If a clear use case emerges with proper semantics

Validation

• ✅ All 9 modified submissions verified with cape submission verify
• ✅ All 222 tests passing with cabal test
• ✅ All files formatted with treefmt

Fixes #142

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