feat(allowlist): add Solidity / Web3 (.sol, .vy) support

Tracking parent issue: #470

## Scope

Add the **Solidity / Web3** language group to the review allowlist:

- Extensions: `.sol` (Solidity), `.vy` (Vyper)
- Test exclude patterns: `**/test/**/*.sol` (Hardhat), `**/*.t.sol` (Foundry)
- Rule docs: `internal/config/rules/rule_docs/solidity.md` and `vyper.md`
- Register `**/*.sol` -> `solidity.md` and `**/*.vy` -> `vyper.md` in `system_rules.json`
- Tests: `allowed_ext_test.go` and `system_rules_test.go`

## Acceptance

- [x] `make test` passes
- [x] `make check` passes

Start with allowed_ext_test.go and system_rules_test.go, then inspect system_rules.json and the rule documentation paths under internal/config/rules/rule_docs/. Add the Solidity and Vyper extensions, test exclude patterns, rule registrations, and docs described in the issue; done means make test and make check pass.

Written by the indexing model from the issue text.