feat: add schema version option to init and pack commands

[felixrieseberg]

Introduces a —schema-version flag for init and pack commands to allow users to specify which manifest schema version to use. Sets version 0.2 as the default since no clients currently support version 0.3.

• Added --schema-version option to mcpb init and mcpb pack commands
• Created DEFAULT_MANIFEST_VERSION constant set to "0.2"
• Updated LATEST_MANIFEST_VERSION to remain at "0.3" (newest schema)
• Both commands now default to schema version 0.2 instead of 0.3
• Added validation to ensure only valid versions (0.1, 0.2, 0.3) can be specified

Usage

# Create manifest with default 0.2 schema (client-supported)
mcpb init

# Create manifest with specific version
mcpb init --schema-version 0.3

# Pack with default 0.2 schema
mcpb pack

# Pack with specific version  
mcpb pack --schema-version 0.1

[joan-anthropic]

we'll probably want to update this so that the contents actually reflect the schema for the particular manifest version (ie, no localization field pre-v0.3)