[codex] Centralize release install resolution

[halkn]

What changed

• Introduced InstallPlan to carry resolved release installation data
• Centralized external release resolution in resolver.py
• Updated commands.py to compare and execute updates based on resolved plans
• Simplified installer.py so release installs consume resolved plans instead of re-fetching release metadata
• Reduced duplicate GitHub release access when auto-selecting release assets
• Added and updated tests for resolver, commands, and installer flows

Why

The previous flow mixed external release resolution across commands.py, resolver.py, and installer.py. That made the update path harder to reason about and caused redundant release metadata lookups in some GitHub release flows.

Impact

• Clarifies responsibility boundaries between resolution and installation
• Makes future performance work easier by concentrating release lookups in one place
• Reduces duplicate external access for GitHub release resolution when asset selection is automatic

Validation

• uv run pytest tests/test_resolver.py tests/test_commands.py tests/test_installer.py
• uv run ruff check src tests