[Good first issue] Add PowerShell tab completion

[Xsoleils]

Problem

SoleilCode users on Windows must remember every command and option. PowerShell tab completion would make the CLI easier to discover and gives a new contributor a focused, low-risk entry point.

Proposed experience

Add a PowerShell completion script for the soleil command.

Acceptance criteria:

• Complete top-level commands such as doctor, setup, language, run, and bench.
• Complete documented options including --cwd, --mode, --language, --yes, --json, --suite, and --runs.
• Add short installation and removal instructions to the README.
• Keep the generated/completion file deterministic and add a focused test where practical.
• Do not read credentials or change the free-first routing policy.

Free-first policy

• This proposal preserves SoleilCode's free-first and approval-by-default principles.