Fix BTP CLI syntax errors in subaccount-roles-import skill

[tsvetanmadanski-sap]

Summary

• Fix CF_SUBACCOUNT_ID assignment with os.path.exists() guard to prevent silent failure when config file is absent
• Fix btp list security/app field names: .name/.appId → .xsappname/.appid (lowercase)
• Fix btp add security/role syntax: replace non-existent flags with correct positional args and flags
• Add handling for read-only role collections created by authentication-xsuaa
• Fix inconsistent appId camelCase in jq verification commands
• Update btp-cli-reference SKILL.md and reference files to match corrected syntax

Test plan

• Verify btp add security/role command syntax against BTP CLI v2.106.1
• Verify btp list security/app returns .xsappname and .appid fields
• Test skill execution when .migration/cf-migration-config.json is absent

[cla-assistant]

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