feat: add external YAML formatter integration (--formatter)

[RicardoAGL]

Summary

Adds a --formatter CLI option to refactor, organize, and document commands that runs an external YAML formatter on files written by osmosis. This lets teams use their preferred formatter (prettier, yamlfmt, yq, etc.) in a single command instead of the common 3-step CI pattern (osmosis → formatter → git diff).

• New --formatter flag accepts any command that takes file paths as trailing args
• Written file paths tracked via YamlRefactorContext._written_files
• Formatter runs once after all writes complete (batch, not per-file)
• Failure is non-fatal — logs a warning but doesn't fail the command
• 120-second subprocess timeout (configurable)
• Config precedence: CLI flag > dbt-osmosis.yml > None (backward compatible)

Example usage

# With prettier
dbt-osmosis yaml refactor --formatter "prettier --write"

# With yamlfmt
dbt-osmosis yaml refactor --formatter "yamlfmt"

# With yq
dbt-osmosis yaml refactor --formatter "yq -i '.'"

CI simplification

Before (3-step):

- run: dbt-osmosis yaml refactor
- run: prettier --write "models/**/*.yml"
- run: git diff --quiet models/

After (2-step):

- run: dbt-osmosis yaml refactor --formatter "prettier --write"
- run: git diff --quiet models/

Files changed (12 files, +713 lines)

File	Change
src/dbt_osmosis/core/formatting.py	New — run_external_formatter() with shlex parsing, timeout, non-fatal error handling
src/dbt_osmosis/core/settings.py	formatter field, _written_files tracking, resolved_formatter property
src/dbt_osmosis/core/schema/writer.py	written_file_tracker callback in _write_yaml() / commit_yamls()
src/dbt_osmosis/core/sync_operations.py	Pass tracker through to _write_yaml()
src/dbt_osmosis/core/restructuring.py	Pass tracker through to _write_yaml() / commit_yamls()
src/dbt_osmosis/cli/main.py	--formatter Click option + _run_formatter_if_configured()
src/dbt_osmosis/core/osmosis.py	Re-export run_external_formatter
tests/core/test_formatting.py	New — 15 tests (success, no-op, failure, timeout, command parsing, yq, yamlfmt, frozenset)
tests/core/test_settings.py	10 new tests (formatter settings, written file tracking, resolved_formatter)
tests/core/test_schema.py	2 new tests (writer tracker callback wiring)
tests/core/test_cli.py	1 new test (--formatter in help output)

Test plan

• 692 unit tests pass (0 failures, 6 pre-existing skips)
• Integration-tested with prettier 3.8.1 against demo_duckdb — formatter ran on 4 files successfully
• Integration-tested with yq 4.52.2 against demo_duckdb — formatter ran on 4 files successfully
• Tested in production dbt project (dbt-core 1.11.2 + dbt-databricks 1.11.4) with prettier
• Security review: approved (subprocess timeout, no shell=True, shlex parsing)
• Backward compatible — without --formatter, behavior is identical to current

Dependencies

Note: This PR is stacked on #317 (feature/fusion-compat). The 3 formatter-specific commits are:

• 0699e88 feat: add external YAML formatter integration (--formatter)
• 220ca76 fix: address security review findings for external formatter
• 57900b4 fix: resolve written file paths to absolute for external formatter

Once #317 is merged, this PR's diff will show only the formatter changes.