config: update fields, defaults, and required checks to match README

[yushan8]

Summary

Aligns config field names, defaults, and required checks with config/README.md from #180.

• Renamed fields: worker_pool_size → max_worker_pool_size, repo_manager_clone_path → workspaces_root, chunking → streaming (with sub-fields renamed to max_num_targets/max_num_changed_targets/max_num_metadata_entries), query_timeout → query_timeout_seconds.
• config.Parse now enforces service.workspaces_root as required and applies documented defaults: bzlmod_enabled defaults to true, query_timeout_seconds defaults to 600, streaming fields default to 250/125/50000.
• Added Go doc comments for config fields documented in the README.
• Updated all callsites and YAML fixtures for the renames.
• Added config/config_test.go covering new validation and defaults.

Test plan

• make build
• make test

Stack

1. [config] Tango configuration documentation #180
2. @ config: update fields, defaults, and required checks to match README #190
3. config: remove fields not documented in config/README.md #191

[liujsj]

Why is this not in the const block too?

[yushan8]

It's because the value is *bool not bool. Since the default is now true we need to make sure the value was unset. If we made it a bool, the default is false. There is no way to tell if it's false because it's unset or false because the user set it. So to check if bzlmod was disabled, we would check BzlmodEnabled != nil && *BzlmodEnabled == false.

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