Improve AGS CLI DX guardrails

[keithfx999]

Summary

• tighten CLI error handling, config validation, backend selection, and non-TTY root behavior
• align E2B run/exec/file flows around a shared sandbox helper with clearer API errors and token handling
• update docs plus local DX audit records for discovered CLI/documentation issues

Tests

• go build -o /tmp/ags-dx-fix .
• go test ./...
• go vet ./...
• golangci-lint run ./...
• git diff --check

Smoke checks

• non-TTY root command prints help instead of panicking
• JSON error mode returns structured missing-key errors
• fake E2B key surfaces E2B auth failure instead of Cloud SDK signature failure
• local flag validation catches invalid run/mobile/browser inputs before network work