Add configurable program switch policy

[mad-sol-dev]

Summary

• add an environment-driven program switch policy to gate mid-session selection changes
• surface soft-policy warnings in program selection tools/routes and response schema
• document the new policy and refresh OpenAPI snapshot and tests

Testing

• python -m pytest bridge/tests/unit/test_program_context.py bridge/tests/unit/test_program_selection_tools.py
• UPDATE_SNAPSHOTS=1 python -m pytest bridge/tests/golden/test_openapi_snapshot.py::test_openapi_snapshot_drift

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Codex Task