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/.solhint.json

@@ -0,0 +1,20 @@
+{
+  "extends": "solhint:recommended",
+  "rules": {
+    "compiler-version": ["error", "^0.8.0"],
+    "func-visibility": ["error", {"ignoreConstructors": true}],
+    "state-visibility": "error",
+    "max-line-length": ["warn", 160],
+    "quotes": ["error", "double"],
+    "reason-string": ["warn", {"maxLength": 64}],
+    "no-empty-blocks": "warn",
+    "no-unused-vars": "warn",
+    "no-console": "off",
+    "code-complexity": "off",
+    "function-max-lines": "off",
+    "gas-custom-errors": "warn",
+    "gas-calldata-parameters": "warn",
+    "not-rely-on-time": "off",
+    "avoid-low-level-calls": "off"
+  }
+}