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/package.json

@@ -1,5 +1,19 @@
 {
   "dependencies": {
     "@uniswap/universal-router": "^2.0.0"
+  },
+  "devDependencies": {
+    "husky": "^9.1.7",
+    "lint-staged": "^16.2.3",
+    "solhint": "^6.0.1"
+  },
+  "scripts": {
+    "lint:sol": "solhint 'src/**/*.sol' 'script/**/*.sol' 'test/**/*.sol'",
+    "lint:sol:fix": "solhint --fix 'src/**/*.sol' 'script/**/*.sol' 'test/**/*.sol'",
+    "format:sol": "forge fmt",
+    "format:sol:check": "forge fmt --check",
+    "lint": "npm run lint:sol && npm run format:sol:check",
+    "lint:fix": "npm run lint:sol:fix && npm run format:sol",
+    "prepare": "husky"
   }
 }