Add multi-account support

[GrigoriLab]

Summary

• Adds support for multiple named LinkedIn accounts with --account <name> global flag
• New account command with list, add, remove, default, migrate, and whoami subcommands
• Backward compatible: existing single-account configs continue to work unchanged
• Zero changes to existing 16 command files — account selection flows transparently through resolveCredentials()

Changes

• src/lib/types.ts: Added AccountConfig interface; extended LinkedConfig with accounts? and defaultAccount?; added account? to AuthOptions
• src/lib/account-context.ts (new): Module-level get/set for active account name
• src/lib/config.ts: Added getAccountCredentials, listAccounts, setAccount, removeAccount, setDefaultAccount, migrateLegacyCredentials
• src/lib/auth.ts: Updated credential resolution priority to be account-aware (explicit cookies > env vars > named account > legacy flat config > browser extraction)
• src/commands/account.ts (new): Full account management subcommands with input validation
• src/cli.ts: Added global -a, --account <name> option with preAction hook to set context
• src/index.ts: Exported new functions and AccountConfig type

Test plan

• pnpm build — TypeScript compiles clean
• pnpm test — All 198 tests pass (27 new tests for account functionality)
• pnpm lint — No lint errors in changed files
• Unit tests for account-context get/set/reset
• Unit tests for config CRUD (listAccounts, setAccount, removeAccount, setDefaultAccount, migrateLegacyCredentials)
• Unit tests for account-aware auth resolution (named accounts, default accounts, env var priority, error messages)

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