Add Solidity linting with solhint, Foundry formatter, and pre-commit hooks (#51)

## Changes

### Configuration
- Added .solhint.json with recommended rules + custom config
  - 160 char line length (warn)
  - Double quotes enforcement (error)
  - Explicit visibility required (error)
  - Console statements allowed (scripts/tests need them)
  - Gas optimization warnings enabled
  - Ignores test/helpers/, lib/, out/, cache/, broadcast/

- Added foundry.toml [fmt] section
  - 160 char line length
  - 4-space tabs
  - Double quotes
  - Thousands separators for numbers
  - Sort imports enabled

- Added .lintstagedrc.json for pre-commit auto-fix
  - Runs solhint --fix on .sol files
  - Runs forge fmt on .sol files

- Added husky pre-commit hook via lint-staged

### NPM Scripts
- lint:sol - run solhint
- lint:sol:fix - auto-fix solhint issues
- format:sol - format with forge fmt
- format:sol:check - check formatting
- lint / lint:fix - combined commands

### Code Changes
- Added explicit visibility modifiers (internal) to constants in scripts and tests
- Fixed quote style in DeployLocal.sol
- All Solidity files formatted with forge fmt

## Verification
- ✅ forge fmt --check passes
- ✅ No solhint errors (warnings only)
- ✅ forge build succeeds
- ✅ forge test passes (107/107)

resolves #44

Co-authored-by: johba <johba@harb.eth>
Reviewed-on: https://codeberg.org/johba/harb/pulls/51

onchain/test/mocks/MockStake.sol

@@ -9,7 +9,7 @@ pragma solidity ^0.8.19;
 contract MockStake {
     uint256 private _percentageStaked;
     uint256 private _averageTaxRate;
-    
+
     /**
      * @notice Set the percentage staked for testing
      * @param percentage Value between 0 and 1e18
@@ -18,7 +18,7 @@ contract MockStake {
         require(percentage <= 1e18, "Percentage too high");
         _percentageStaked = percentage;
     }
-    
+
     /**
      * @notice Set the average tax rate for testing
      * @param rate Value between 0 and 1e18
@@ -27,7 +27,7 @@ contract MockStake {
         require(rate <= 1e18, "Rate too high");
         _averageTaxRate = rate;
     }
-    
+
     /**
      * @notice Returns the mocked percentage staked
      * @return percentageStaked A number between 0 and 1e18
@@ -35,7 +35,7 @@ contract MockStake {
     function getPercentageStaked() external view returns (uint256) {
         return _percentageStaked;
     }
-    
+
     /**
      * @notice Returns the mocked average tax rate
      * @return averageTaxRate A number between 0 and 1e18
@@ -43,4 +43,4 @@ contract MockStake {
     function getAverageTaxRate() external view returns (uint256) {
         return _averageTaxRate;
     }
-}
+}