feat: Add auto_index_on_activate config option

[w0wl0lxd]

Summary

• Adds new auto_index_on_activate config option to ProjectConfig (defaults to false)
• When enabled, automatically indexes all project source files after project activation
• Pre-populates symbol caches for faster first queries on large projects

Changes

• Added auto_index_on_activate: bool = False field to ProjectConfig dataclass
• Implemented _auto_index_project() method in SerenaAgent that runs after LSP initialization
• Updated project.template.yml with documentation for the new option
• Added 3 unit tests for config parsing behavior

Test plan

• [x] Run uv run poe format - passes
• [x] Run uv run poe type-check - passes
• [x] Run uv run poe test - all 10 config tests pass
• [ ] Manual test: Enable option in a project.yml and verify indexing runs on activation