Honor persistent flags in proxy commands

[gtsiolis]

Generalizes #273. Proxy commands set DisableFlagParsing, so lstk's persistent flags were forwarded to the wrapped binary (rejected as unknown options) and their effect silently dropped. --config is broken on lstk aws the same way --non-interactive was — it's ignored and passed through to aws.

Changes

• Add stripGlobalFlags, which extracts --non-interactive and --config (both --flag value and --flag=value, in any position) from a proxy command's args. Reusable by future proxy commands; replaces the one-off stripNonInteractiveFlag.
• lstk aws now sets cfg.NonInteractive so interactivity is consistent across the codebase, and routes --config back through Cobra so initConfig resolves the overridden path instead of forwarding it to aws.

Tests

• Unit TestStripGlobalFlags (both flags, both forms, interleaving, edge cases).
• Integration TestAWSCommandStripsGlobalFlagsFromPassthrough — asserts both flags are consumed by lstk, not forwarded to the aws binary.

🤖 Generated with Claude Code

[gtsiolis]

Follow up from #273 (review). Cc @carole-lavillonniere

Possibly worth waiting for #251 to land first.