Add Muse Code harness support

[JacobStephens2]

Adds Muse Code as a standard Harness so va muse launches with the selected Manifest and va muse --yolo forwards Muse's native flag unchanged. The default is bare muse in the caller's directory, preserving Muse's permission settings and native resume arguments.

Includes the shipped profile, installer discovery and existing-profile preservation, Doctor working-directory checks, operator documentation, and CLI acceptance tests. Uses the existing secret-resolution and Manager-token removal path.

Closes #103.

Validation:

• cargo test
• cargo check --all-targets
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• bash -n install.sh
• A real Muse 1.1.1 shell launched through the built Launcher with exec --yolo received an injected non-secret sentinel; the tool result reported the expected output and exit code 0. No real third-party credential or authenticated external action was tested.

Standards and Spec reviews completed: no correctness or spec findings; one optional note about the installer's existing repeated discovery pattern was left for a broader refactor.