Add Prime feature flag evaluation client

[d42me]

Adds a Prime CLI feature flag client that evaluates defaults via the platform feature-flags endpoint with CLI version and team context, falling back to defaults when evaluation is unavailable.

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Note

Low Risk
Low risk: adds a new optional API call with defensive validation and a default fallback path, plus unit tests; no existing flows are modified beyond new exports.

Overview
Adds prime_cli.feature_flags with FeatureFlagsClient.evaluate() to fetch evaluated flag values from /feature-flags/evaluate, sending cli_version and optional team_id, and falling back per-key to supplied defaults when the API omits values.

Exposes FeatureFlagsClient, evaluate_feature_flags, and is_feature_enabled from prime_cli.__init__, and adds unit tests covering request payload construction, empty-default short-circuiting, API failure fallback, and strict boolean handling in is_feature_enabled.

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