Restructure /init-deep command prompt with dynamic phases and concurrent execution

- Reduce phases: 5 → 4 (discovery, scoring, generate, review)
- Implement concurrent execution: fire background explore agents + LSP simultaneously
- Add dynamic agent spawning based on project scale (files, lines, depth, large files, monorepo, languages)
- Convert to telegraphic style: ~50% shorter (~427 → ~301 lines)
- Clarify --create-new behavior: read existing → delete → regenerate

Addresses issue #368 requirements for dynamic agent spawning and concurrent explore+LSP execution.

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Author: code-yeongyu