Replace Dynaconf with pydantic-settings-driven configuration loading

[sagikimhi]

Motivation

• Remove runtime dependence on dynaconf and rely on a lighter, explicit configuration stack that uses pydantic-settings for env-aware values and python-box for nested/dotted access and serialisation.
• Provide a simple, maintainable loader that reads YAML/TOML/JSON, supports include chaining and jinja template expansion, and preserves existing settings access patterns across the codebase.

Description

• Removed dynaconf from runtime dependencies and updated README/pyproject.toml to reflect the new stack (pydantic-settings + python-box).
• Replaced the Dynaconf-backed Settings with a new Settings implementation in src/socx/config/_settings.py that uses box.Box for in-memory mapping, pydantic-settings for environment integration (SettingsEnv), supports settings_file/preload, load history, load_file (YAML/JSON/TOML), and @jinja template resolution.
• Updated configuration bootstrap in src/socx/config/_config.py to use the new ensure_a_list and Settings class and preserved the LocalProxy settings behavior and user/local override loading flow.
• Reworked serializers/encoders/validators to operate on the new Settings type and box.Box (src/socx/config/serializers.py, src/socx/config/encoders.py, src/socx/config/validators.py).
• Replaced Dynaconf-specific converter plumbing with a local compatibility layer in src/socx/config/converters.py (introduces a lightweight Lazy shim and internal converter registry) and migrated callers (e.g., src/socx_plugins/git/summary.py) to use the new converters/API.
• Replaced Dynaconf utilities usages across plugins: swapped DynaBox with box.Box, replaced Dynaconf file helpers with stdlib equivalents (e.g., glob) and local dedupe logic in src/socx_plugins/git/utils.py, and updated config CLI/history rendering to use box.Box.

Testing

• Ran python -m compileall src/socx/config src/socx_plugins/git src/socx_plugins/config, which completed successfully.
• Attempted to run PYTHONPATH=src python -m pytest tests/test_regression_loading.py -q, but test execution could not complete in this environment because package metadata for socx-cli is not installed (importlib.metadata.PackageNotFoundError), so full pytest validation was not possible here.

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Codex Task