fix: address code review issues from alpha.14 to alpha.15

[alexeyv]

Summary

Adversarial code review of changes between v6.0.0-alpha.14 and v6.0.0-alpha.15 identified 10 issues. This PR addresses 8 of them (2 were intentionally ignored as by-design).

Fixed

• Welcome to BMAD-METHOD Discussions! #1 Remove debug console.log statements from ui.js
• So far EXCELLENT! #2 Use consistent return type in handleMissingCustomSources()
• Combining BMAD-Method with cursor-custom-agents-rules-generator #3 Hoist CustomHandler require to top of installer.js and ui.js
• My BMAD-Method Prompts #5 Add error handling and rollback for temp directory cleanup
• Custom mode for PO missing some context? #6 Fail fast on malformed custom module YAML (throw instead of silent continue)
• refactor: update story naming to match epic:story scheme from PRD and fix typos #7 Clone config at install() entry to prevent caller mutation
• Cursor Custom Modes? #8 Use streaming for hash calculation to reduce memory usage
• Context Handling in Cursor vs. Windsurf: Seeking Insights #9 Remove duplicate CustomHandler import in ui.js (fixed with Combining BMAD-Method with cursor-custom-agents-rules-generator #3)

Ignored (by design)

• Existing Project Integration #4 CSV parsing - closed contract; we control both writer and parser
• Project integration #10 URL feature placeholder - intentional; low priority future feature

Test plan

• npm run test passes (schemas, install, bundles, lint, format)
• Manual test: fresh install
• Manual test: quick update on existing installation

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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