feat: bitrefill init for OpenClaw and credential store

[marcopesani]

Summary

Adds bitrefill init to validate an API key, store it in ~/.config/bitrefill-cli/credentials.json (0600), and optionally register Bitrefill as an MCP server for OpenClaw with ${BITREFILL_API_KEY} in openclaw.json (no plaintext key in config). Generates ~/.openclaw/skills/bitrefill/SKILL.md when OpenClaw is detected or --openclaw is passed.

Changes

• Credential store: resolveApiKeyWithStore() — flag > env > stored file; chmod after write so permissions stay 0600 on overwrite.
• Init: masked interactive prompt, MCP tools/list validation, ~/.openclaw/.env merge, openclaw.json merge, redacted summary.
• CLI: early init handler; init --help / init -h show dedicated help without running init.
• Docs: README quick start, OpenClaw + Telegram, key resolution order.
• Chore: .env.local in .gitignore, .cursorignore for env files.

Testing

• pnpm test (41 tests)
• pnpm build, pnpm format

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