Add flow diff-contract command

[janezpodhostnik]

Closes #???

Description

• Add new flow diff-contract command that compares a local Cadence contract (file or URL) against a deployed version on-chain
• Performs import substitution via flow.json before diffing so comparisons are apples-to-apples with on-chain code
• Outputs unified diff (git patch style) when contracts differ
• Supports --quiet / -q flag for CI usage: suppresses output and exits with code 1 if contracts differ
• Address argument is optional — resolves from flow.json deployments or contract aliases when omitted

Usage

  flow diff-contract <file-or-url> [address] [flags]

  # Diff local file, resolve address from flow.json
  flow diff-contract ./MyContract.cdc --network testnet

  # Diff with explicit address
  flow diff-contract ./MyContract.cdc 0xf8d6e0586b0a20c7

  # Diff from URL
  flow diff-contract https://raw.githubusercontent.com/.../MyContract.cdc my-account --network testnet

  # CI mode: silent, exit code 1 if different
  flow diff-contract ./MyContract.cdc -q

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

⚠️: No snapshots were found for the head SHA 635da08.

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Scanned Files

None

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.06838% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/diffcontract/diff-contract.go	76.72%	19 Missing and 8 partials ⚠️
cmd/flow/main.go	0.00%	1 Missing ⚠️

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