Add lnk create command for empty files and directories

[polymorcodeus]

Summary

Adds lnk create, a shortcut for creating empty files or directories and immediately tracking them in the lnk repo. This removes the need to create files manually before running lnk add.

What's new

• New lnk create command - creates empty files or directories and tracks them in one step.
• Directory placeholder - empty directories are created with a .lnkkeep file so git tracks them.
• Dual directory syntax - directories can be requested with --dir or a trailing path separator.
• Host scope support - works with --host for per-machine creation.

Usage

Command	Purpose
create <path...>	Create empty files and track them
create --dir <path...>	Create empty directories and track them
create <dir>/	Same as --dir using trailing slash
create --host <host> <path...>	Create and track in a host scope

Behavior changes / guardrails

• Invocations must be homogeneous; mixing files and directories in one command errors.
• Existing files and non-empty directories are rejected with a suggestion to use lnk add.
• Targets inside git repos, inside already-managed directories, or already tracked paths are rejected.
• service/create.go adds CreateOptions and a .lnkkeep helper; internal/lnkerror gains ErrPathExists and ErrMixedCreateTypes.

Testing

• Unit tests in service/create_test.go and cmd/create_test.go cover file creation, directory creation, host scope, v1 format, guardrails, and CLI argument validation.
• README and AGENTS updated to document the new command.
• make check runs fmt, vet, lint, unit, and integration suites; all green.

Closes#7