mz-feature-flags

# Feature flags Choosing whether a new feature flag defaults on or off in production is a product decision that needs context an agent rarely has. When adding a flag, ask a human which default to use. When reviewing a PR, do not question the default it chose. However, in tests, a new flag should usually be on, so that sqllogictest and testdrive exercise the new code path. If its production default is off, turn it on for CI in `misc/python/materialize/mzcompose/__init__.py` (`get_minimal_system_parameters` if tests fail with it off, otherwise `get_variable_system_parameters`), which `bin/sqllogictest` and mzcompose's `Materialized` service pass to environmentd as system parameter defaults. Rust integration tests use `TestHarness::with_system_parameter_default`. A value listed there overrides the compiled-in default in CI, so changing a default means changing both. `bin/lint-test-flags` fails unless every new dyncfg is registered for parallel workload and mzcompose (see the mz-debug-ci skill). A compiled-in default is not evidence of the production value: check whether `test/launchdarkly-flag-consistency/mzcompose.py` lists the flag in `INTENTIONAL_LD_OVERRIDES` or `KNOWN_CROSS_ENV_DIVERGENCES`. A session variable takes a system default only if `SESSION_SYSTEM_VARS` in `src/sql/src/session/vars.rs` lists it. Otherwise `--system-parameter-default` for it only logs a warning at startup. Observed: the LaunchDarkly MCP `create-flag` tool creates boolean flags that serve true in every environment, including production. A human must set each environment's off variation and fallthrough to false.