docs: SKILLS.md quality improvements

[mejango]

Summary

• Permissions column added to all Key Functions tables -- each function row now specifies its access control (Permissionless, Split operator, Loan NFT owner, Contract owner, Terminal callback, or View). Verified against source contract modifiers.
• Visibility column added to Storage tables -- each mapping/variable shows public (auto-getter) or internal (no auto-getter, use explicit view function). Cross-referenced with REVDeployer.sol and REVLoans.sol declarations.
• "Reading Revnet State" section added -- quick-reference tables for the view functions an integrator or agent would call to check current stage, split operator, token supply, surplus, auto-issuance remaining, loan details, and deployer config.
• Gotcha Refactor/router terminal permissions #17 trimmed -- the loan fee model gotcha previously duplicated details already present in the Constants table. Consolidated to a brief cross-reference.

Test plan

• Verify all Permissions column values match source access modifiers
• Verify all Visibility column values match Solidity declarations
• Confirm Reading Revnet State function signatures compile against interfaces

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