feat: add sync command for config-driven multi-contract ABI syncing

[satoshai-dev]

Summary

Closes #17.

• Add abi-cli sync command that reads abi.config.json or abi.config.ts to declaratively fetch and write ABIs for multiple contracts
• Support per-contract network overrides, partial failure handling with summary report, and jiti-powered TypeScript config loading
• Export loadConfig, validateConfig, AbiConfig, and ContractEntry from the programmatic API

Changes

File	What
src/config.ts	New — types, validation, config file discovery & loading (jiti for .ts)
src/commands/sync.ts	New — syncCommand with --config/-c flag
src/cli.ts	Register sync subcommand
src/index.ts	Export config types and functions
package.json	Add jiti dependency
tests/unit/config.test.ts	15 tests for validateConfig and loadConfig
tests/unit/sync.test.ts	7 tests for sync command behavior
README.md	Document sync command, config schema, flags, programmatic API
.changeset/add-sync-command.md	Minor changeset

Test plan

• pnpm test — 60/60 tests pass (6 test files)
• pnpm build — clean ESM, CJS, DTS output
• pnpm typecheck — no type errors
• Manual: create abi.config.json in repo root, run node dist/cli.js sync, verify files written to outDir

🤖 Generated with Claude Code