refactor(config): simplify local config creation and loading (fixes #667)

[mickume]

Summary

Closes #667

• Local config.toml is now opt-in via --config flag (replacing --force)
• When local config exists, it is the sole config source — global ~/.agent-fox/config.toml is ignored entirely
• Local config template uses the same promoted-fields style as the global config
• Removed config merge logic from init_project.py and the save/restore hack from init_cmd

Changes

File	Change
packages/af/af/init.py	--force → --config; local config creation opt-in
packages/agentfox/agentfox/core/config_gen.py	generate_local_config_template() uses promoted-fields style
packages/agentfox/agentfox/core/config.py	_load_config_global_local() checks local first; sole source when present
packages/agentfox/agentfox/workspace/init_project.py	Removed force param and merge_existing_config call
docs/cli-reference.md	Updated init docs for --config and config precedence

Tests

• Updated ~15 tests in test_global_config_loading.py for sole-source semantics
• Updated init integration tests for opt-in local config
• All 5857 tests pass (1 pre-existing meta-test failure unrelated)

Verification

• All existing tests pass: ✅
• Linter / formatter: ✅
• No regressions: ✅

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